May I say that I don't really get this "sister" impulse that keeps people from romancing certain LIs? Maybe it's because I have a younger sister, and she's never needed my protection.
For me, it's less of a 'sisterly' thing- I'd probably be protecting other people from MY sister 
It's more of a feeling of 'taking advantage' of dependent/submissive/inexperienced-seeming personalities. My personal tastes run to people with very strong personalities, who are independent, because if I'm honest I'd probably walk all over anyone who let me and that would be an awful thing to do to someone. So the little bit of myself that gets projected in when I play games, even when attempting to play a personality very different to my own is always completely put off by 'innocent' available LI's.
But I think a lot of it has to do with personal taste, there are a lot of people who prefer to be protective and have people depend on them, and they might see the stronger characters as more 'family' and the gentler ones as romance material. Lots of people in the world 