1) I don't really mind it, but I don't actively seek it out. I'm an only child though, so I don't really understand sibling relationships at all. I don't get the icky feeling that some people might do when they relate it back to their real life siblings. I do actually really enjoy twincest in BL manga XD
2) Yes, I think it's absolutely fine to portray it! I love my fiction to break all kinds of taboos, since it's just that - fiction. It's a way to explore things that would/should never happen in the real world. There's a lot of fiction out there that features incest (Game of Thrones being just one example), but I do think that VN have a tendency to normalize it and make sibling-love all shiny and happy. I don't have anything against that, it's just interesting how it's become a 'thing' in many eroge/hentai, and has even seeped into non-erotic manga and anime (Oreimo etc.).