You don’t hook up with your girlfriend’s sister unless you’re looking to napalm both relationships.
Your reasoning for not being with the sister is only that she was with someone at the time so you settled for the other one (your current gf)? Is that it? Cause that’s what you’re making it sound like. Not a good reason to be with someone. Ever. You’re being dishonest with yourself and you’re betraying the one you’re with. No one should ever be second best in their partner’s eyes. You don’t do that to someone you care about.
If this is how it feels to you, that you’ve basically settled for one because the other one was taken, then I should think the answer is fairly obvious. Break up with your girlfriend and leave the family alone. Do not go after the sister. If you care about either of them, don’t do that to their family, don’t divide them that way. Are you truly in love with either of them? I don’t think so. If you’re going to break up with one and choose the other, you better be damn well sure that you’re in love. That it’s worth it. This is not some silly game of who gets you hotter!
Besides dreams aren’t portents. Just because you dream about sex with this girl, doesn’t mean you’re in love with her or that you have to be with her or that you even WANT to be with her! It seems more of a sign that your current relationship is having issues – not that you have actual feelings for her sister. Dreams are rarely so literal as that.
Maybe this fixation on her sister is just an easy way to just blow it all up and walk away without having to deal with the consequences in a mature, rational way. Maybe you’re actually falling for her sister. I don’t know the answer, but you do. You need to address the problems in your relationship – don’t just throw a sister-shaped bomb at it and run away.
Do the right thing.
Sumptuous Daydreams 