And risk tearing apart a family?
No, that's just heartless.
It sounds like you're unhappy with your current gf, so waiting for things to play and see if you guys stay together in a content relationship that may or may not end it marriage or your relationship dies out and you move on with your life, but don't get with her sister, or you may be making a mistake. These feelings sound like infatuation and not true love at all. That's why you think it sounds a little like high school.
Who knows, maybe you'd find that your gf is really more deep than you thought if you spent more time with her and not her sister, but she might be holding back because she's afriad of losing you. Some people do that because they aren't sure people could love them for who they are. Good luck OP