Wouldn't it make more sense if Arthur and Morgause had the same mother (Igraine) based on all the look-alike references? That'd be (chronologically):

Igraine + Gorlois = Morgause
unknown + Gorlois = Morgana
Igraine + Uther = Arthur

To be perfectly honest, the half-sister connection between Morgana & Morgause bothers me a bit because it messed up my original theory about Morgana's parents, but oh well. THANK YOU, WRITERS! :p