It does seem like it would have been a lot easier to just make Morgana + Morgause full sisters, eh? But I suppose half-sisters are more dramatic in the sense of storytelling.

I had a theory that Igraine and Morgause were half sisters also; sharing the same mum, while Morgana and Morgause shared the same father. But, as you said, it does leave a lot of characters to create. Still, that's more or less the basic idea I had for my canon (or what I plan on writing for my fic).