Morgause's Parents 

It was recently learned that she has a half sister, Morgause, who was smuggled out of Camelot by Gaius at her birth, and it was revealed by Gaius that she is the daughter of Gorlois. The identity of her mother remains unknown.

We can't know Gorlois is Morgause's father for sure, just from the crest she wore and in fact it seems like its the other way around, from recent events. Helen (I believe is her name?), Gorlois' wife, seems to be the shared parent of Morgana and Morgause.

Spoilers for 3x05, if you haven't seen:

If Morgana and Morgause share a father, why did Morgana continue to call Morgause 'sister' after revealing to Morgause that Uther was her father? They would be entirely unrelated if Gorlois was not Morgana's father. And you would think the subject would have come up.

Also, the way Morgause mentioned that the healing bracelet she gave Morgana was 'a gift from her mother' seems to imply more that they share a mother and not a father.