RurouniZel
Asks questions so Ezalc doesn't have to
riiiiiiight
because feeding your sisters perversion, taking blame for it, and dealing with constant stress brought on by her wacky perverted hijinks and batshit crazy friends isnt a doormat character?
His only purpose in the show is to deal with constant shit because hes attracted to his sister, which is a pretty terrible one.
Here's how I see it. Kyousuke's basic character motivation (at least at first anyway) is the desire to reconnect with his little sister. For what's apparently been quite some time, probably years, his relationship with Kirino has been one of cold indifference. But after accidentally discovering her hobby, she decides to confide in him. She came to him with a genuine plea of help. And he wants to help, because she is his little sister and no matter how much he wants to deny it, he does love her as such (how much he loves her... well I won't go into where that might be going but at the beginning that's definitely not the case).
The "wacky friends" was actually his idea, because Kyousuke was smart enough to realize that what Kirino was originally trying to do (make him play all of the eroge she plays so she can talk to him about them, etc.) wasn't going to work. She needed friends who were actually hardcore into this stuff at least somewhat close to her level. And over the course of that process they ended up being friends of his as well.
And yes, he's dealing with all of the stress of the situation, but again he's doing so because he doesn't want to lose what he's finally gained after all this time; Kirino's acceptance of him as her brother and not just some stranger living in the same house. He's enjoying the feeling of being needed by his little sister, and doesn't want to throw that away, so yes he does bend a little bit more with her. But there are times where he most definitely puts her in her fucking place (like Episode 11 at the party). Not the behavior of a doormat.
The part where he took the blame for eroge stuff was dumb though, no question (however it was fucking funny).