Whoa, wait a second. "This person isn't only attracted to their sibling"? Who are you to say that? I get your argument but it's pretty ****** up.
For instance, I dated a lot of women that I had no interest in marrying. They were attractive but they were not "the one". It's entirely possible that one could believe, correctly, that only their sibling could be their "life partner".
Using your argument, I could say a gay man needs to give a some women a chance before deciding that he's found "the one" in a dude.