Board Thread:Questions and Answers/@comment-184.88.25.24-20131112235645/@comment-5674726-20140202123254

I don't think that Morgana's belief that the throne should be heard can be taken as evidence of birth order, one way or another.

In Season Three, she and Morgause base their plans around the idea that, if Arthur is out of the picture, Uther will name his only surviving child his heir rather than Morgana thinking that she is the rightful heir and being cheated of her rights. It's only later that she thinks of the throne as hers so I'd say that this belief was based on a combination of her mental instability - she wanted the throne, therefore it was hers - and the fact that she was crowned Queen and had the throne taken from her.