If nothing else, the author has made doujinshi were Rin and Kyousuke (who are very much blood-related) from Little Busters are doing it with each other, so he's certainly into actual brother/sister incest. Of course, there's still the possibility that he gets pressured into making them not blood related. But then, it'd be strange to advertise that they're blood related, as they have apparently done.
Speaking of which, I read that the characters here are based on Rin and Kyousuke. But there's not much similarity, aside from Uta being rather cat-like. Their personalities are completely different, to say nothing of their relationship.