Okay, this needs to be settled.

Jenova is more Sephiroth's mother than whats-her-face(sorry can't remember he name) ever was. Sephiroth grew up thinking that Jenova was his mother and he most likely has never heard of whatsherface. Jenova's DNA probably over powers whatsherface's so much that Sephiroth can be considered more alien than he is human. (call him a hybrid...) That's why he is so powerfull.

I believe that it has been established that it was Sephiroth controlling Jenova to fight you. Good. We understand. Now shut up about it. And though it's Jenova's body you're fighting, Sephiroth is the one pulling the strings. (Nothing like a puppet to tell someone they're a puppet, eh?) So technically, you are fighting him, just not his physical self. OK? Good.