User blog comment:Merlin919/Is Morgause, Vivienne and Gorlois's daughter/@comment-3012002-20120816010259/@comment-5350801-20120816045748

We're dealing with fantasy. Anything is possible in fantasy. Some legends have Modred being Arthur's illigitament son, well others have him a nephew.

Yes it was a crime, but it was done. Uther and Vivienne did it, so why not Gorlois and some other woman. Gorlois could have married Vivienne later and lied and said he had a child (Morgause) from a previous marriage and that he was a widower.

But the fact that Morgause was smuggled outta Camelot as a baby is a strong indicator that her and Morgana had the same mother.

There is just so much unknown about who was born first and second in this case, we can't possibly answer.