How's the manga?
Better than the VN
It's okay. It's a good adaptation of Arc's route with some reasonable inclusions from Ciel's and Akiha's route, as well as some original material that isn't bad.
It completely omits Hisui and Kohaku's stories, though, so don't read it thinking it's a passable alternative to the VN, because it's really not. It also skimps on Satsuki, for that matter.
Does Shiki fuck his sister?
Shiki only gropes 36DD and fucks Arcueid,no incest, I'm afraid.
Well that's good to hear.