During the show, we met many different characters and were introduced to different relationships between them. Merlin and Gaius, one of my favorites, shared a mentor/student one, Merlin and Hunith/Ballinor shared a son/parents relationship, and of course there is the brother relationship between Arthur/Merlin or Arthur/knights of Camelot, and the romances Gwen/Arthur, Merlin/Freya, Merlin/Gwen(season 1), Arthur/Morgana(season 1), Gwen/Lancelot(season 1 and 2) and so on.
Out of all these, I think that the most tragic relationship is between the Pendragon Family. Here's how they evolved throughout the series:
So, in the beginning, everything's perfect, love is in the air (the producers probably did not know they would make them siblings in the future, or simply wanted to play with the incest thing...?) and Arthur and Morgana are introduced as the perfect future loyal couple. Gwen admits that "some are born to be queen", but continues saying "Who would like to marry Arthur?" (how come Uther never heard of the loyals or peasants talk about his two kids marrying each other?). Soon, however, we understand that there is more to meet the eye than a teen romance. Morgana looks out for Arthur, and is distraught whenever she sees a vision of him dead. Arthur becomes her champion and protects her from trouble. Despite their deep care for each other, they make each other jealous at every chance, like when Valiant wanted to accompany Morgana to the feast, or when Arthur wanted to marry Sophia.
However, even from this early point, it is made clear that their future does not want them together. Arthur is annoyed at Morgana whenever she tries to protect him (like from Sophia, or the Questing Beast) and thinks she overreacts. Morgana wants to protect him but he doesn't believe her and she cannot let him know that she has magic. So their apparent "perfect" relationship slowly begins to crumble.
In season 2, the two siblings have only little interaction. Arthur falls in love with Guinevere, and Morgana's adventures with magic have just begun. However, the mutual trust, love and respect between them still exists. Morgana feels safe when she sees Arthur after everyone has fallen asleep, and Arthur tries to protect her at all costs. However, it is clear that their relationship is not the same as before.
In season 3, their relationship is completely ruined. Until "The Crystal Cave", Morgana and Arthur retain their season 1 interaction, despite her deep hatred for Uther. After she learns about her birthrights, though, she pushes Arthur aside and often tries to have him killed, in order for her to have the throne for herself. When Arthur realises this, he is completely broken but he does not look like he has given up on his sister.
One year later, Merlin condemning Morgause to death made Morgana even more bitter towards Camelot. Instead of indirectly trying to cause his death, she steps up and tries to kill him with her own hands. However, when he confronts her, both look evidently hurt and deeply sorry. Arthur questions what Morgana has become, and she says that she thought they were friends, until she realised how much Arthur is like Uther. When Arthur returns the "compliment", Morgana snaps, and their last chance to reconcile forever disappears...
Even in early series 5, though, Arthur has not given up on her. He still wonders what happened to her, but this time she does not hesitate to attack and hurt her brother. Arthur later develops a hatred for her as well, and brother and sister go on a full-out war. Morgana has been obsessed with killing Arthur, but when he is on his deathbed, there is not satisfaction on her face. Same goes for Arthur, when Morgana lies on the ground, pierced by Excalibur. Perhaps in the end they both realised that nothing is more important than family and wanted to make things different...or they were both tired of their fights. I cannot have a better guess...
Overall, the two used to be very close, having grown up together, and Morgana used to be a person that Arthur would sacrifice his throne for. She protected him and he protected her, but the two grew distant, and after Morgana found out about her birthrights, they tried to kill each other.
Who is to blame?
In series 1 and 2, I would say Arthur. He did not believe her warnings and thought she was just jealous or overreacting. He also made her feel alone in her most difficult times. From series 3 and onward, though, I only blame Morgana (Arthur is maybe at fault only for not bringing magic back).
Arthur and Uther
The lovely father/son relationship between Uther and Arthur! Until series 4, this relationship was only made stronger, but unfortunately, Uther returning as a ghost and trying to kill Arthur completely ruined their previous bond.
In series 1, Uther and Arthur did not have the "ideal" relationship. Although at times it was made obvious that Uther genuinely cared for him, like the episode "Excalibur", at other times he was very distant from Arthur and his problems. There were times that he treated Arthur as the best soldier of his kingdom(not that he wasn't) and at times even told him he would not accept failure. Whenever Arthur disobeyed him, like "The Poisoned Chalice", it was made obvious that Uther did not like to be disobeyed, even by his own son. However, his love prevailed most times, like when he cried for Arthur in "Le Morte d'Arthur" or when he protected his son by sacrificing his own life againist Tristan de Bois.
Their relationship becomes more strained in series 2. Morgause's arrival and Arthur's curiosity about his mother causes Arthur to almost kill Uther, his father, but Merlin manipulates him into not doing it. Other than that, their relationship is similar to season 2. In season 3, their relationship follows the same path as well and we see no or little development, since more attention is given to Morgana's relationship with Uther.
In series 4, their dynamic drastically changes. Uther, a broken man, is very concerned and fears that Arthur will leave him too. He cannot bear to let his son go to the Isle of the Blessed when the Dorocha are released, and overall has become a more sensitive and quiet person, spending almost all the time in his room, thinking about Morgana's betrayal. The ultimate test for Uther is to protect Arthur by the Gleeman, when the latter tries to kill him, and he does so, albeit with grave consequences for his own health, as he meets his demise.
The above is all ruined in series 5. One episode is all it took for their relationship to be destroyed. Uther wanted to meet his father more than anyone and went to him in the spirit world. However, all he heard was criticism and how he was destroying the kingdom. Looking back at his father caused his ghost to haunt Camelot and attempt to kill the servant-queen, the commoners-knights, and the sorcerer-servant(Merlin) and even Arthur himself, saying that he is sorry, but the kingdom takes priority. It also left various plot holes, as Uther was already aware in series 4 of the relationship between Arthur and Gwen and the knighting of commoners.
Overall, the two shared a great and complex relationship until series 5, when once again everything was ruined. They used to have their bad moments, but Uther never tried to kill his son. Arthur, in return, blew from the horn, causing Uther to eternally suffer in the underworld...
Who is to blame?
I would say Uther. Arthur never hid from him what kind of king he wanted to become, but Uther suddenly became a ghost-terrorist-murderer-whatever just to change the way Camelot should be ruled. I wonder, is having only Gwen on the throne better for Uther than her ruling alongside his son?
One of my personal favorites. Their relationship is very complicated.
Prior to the beginning of the show, Morgana grew up with Gorlois, ignorant about her true father. When he died at the war, because Uther failed to send reinforcements in time, he took Morgana in, and, according to him, she fought him from the beginning. According to Morgana, however, Uther used to be stubborn and he didn't like being challenged, which caused their relationship to be damaged from the beginning. In addition, Morgana growing up with the supposed murderer of her father, who would have her killed too if he ever found out about her magic(at least that's what she thought) took its toll on their bond.
In series 1, Uther did not show many signs of affection towards Morgana, and she was just his ward. He was strict, and at times brutal when she opposed him. In the very first episode, it was made clear that the two did not get along well, as Uther forced her to participate to an event againist her own will. When she hid Mordred, and Arthur finally managed to get him out of Camelot, Uther threatened her that the consequences would be grave if he finds out she has taken part in it in any way. When she told him that he has blood on his hands, after killing Tom, Uther not only locked her in the dungeons, like he did with Arthur in 1x04, but also tied her up in the wall, making her go through torture to teach her a lesson.
This causes Morgana to conspire againist him and make an attempt on his life. Seeing how he persecutes magical and non-magical beings(when he thinks they are involved with magic) and how he "killed" her father and "it keeps happening", makes her think that he would be better off dead. However, once Uther shows signs of true remorse, and promises her that he will change, Morgana changes her mind and saves his life from her former ally.
In series 2, not much interaction is shown between the two, until the end of the series. Morgana starts exploring her magic and is afraid that Uther will kill her, should he ever find out. This leads her to seek help from other magical beings, like the peaceful Druids, but also the fanatic Alvarr and the mysterious Mordred. She develops romantic feelings for Alvarr, and when he is caught and is to be executed, she and Uther have a huge argument that causes their relationship to break apart. Morgana says that she wants Uther dead once again, and unbeknownst to her, she becomes the source of an enchantment.
When Morgana is taken away from Camelot, Uther spends an entire year searching for her. Apparently a year with Morgause is enough to brainwash her and turn her completely againist Uther. He now develops sign of affection and love for her, but Morgana is very cold againist him. She mentally tortures him and still wants him dead. After she exposes her true self, she keeps him in the dungeons, to "suffer as she has suffered", and executes people of Camelot indiscriminately to "teach him a lesson".
An exception would be the episode "The Crystal Cave". When Morgana learns that she is his daughter and the two are finally about to reconcile, Uther denies to accept her as his daughter. This makes Morgana think that he does not care about him and simply wants to be the perfect king for his people. After this, the last chance for the two to repair their relationship is lost forever.
In series 4 and 5, Morgana denies that she is Uther's daughter, whenever they call her that. In fact, she even manages to kill her father, with the help of Agravaine, the Gleeman and Merlin (unbeknownst to him). After his death, though, she is clearly devestated, despite causing it herself...
All in all, the two never shared an ideal father/daughter relationship, and they kept secrets from each other all the time. Although there were signs that Morgana cared for Uther as well, she became cruel againist him when she realised that his persecutions againist magical beings will never end.
Who is to blame?
Both of them, equally. nothing more needs to be said here.
All in all, the Pendragons, despite caring for one another, have caused great deal of suffering and death amongst each other. Love turns into hatred and family bonds do not matter anymore. You may or may not agree with what I've said, but either way I think that the Pendragon issue needs to be discussed! Closing this post, I think that Ruadan was right, when commenting on the Pendragon "curse"...