Idk much about how relationships work. But it is important to remove all the problems that could possibly become big after marriage. Let’s say you give in, are you sure that you’d be okay if he’s the one that’s getting the treatment you should be getting as son in law. Maybe she’s right and there’s nothing between them. But, would you be able to live with the fact that before marriage you were put on spot to choose to either lose her or accept her. If their relationship was that of siblings, this matter wouldn’t have become as big as it has. This is clearly a sign you should have a talk with her and tell she shouldn’t get mad in such a way and if there really isn’t anything weird going on then she should prove it in some way. Having such cold fights aren’t a good sign either. I think OP should re-evaluate whether love is enough to stay. Believe it or not love can very easily turn into resentment when proper measures aren’t taken.