Not much to do, they are the adults, and they have the problem, they’ll sort it out. If you find small ways to make her life better or show her your support, that won’t hurt, but don’t worry about having to do something.

The thing is though they won’t resolve it themselves I’ve always been the peace keeper between the two of them but I can’t handle it anymore. It puts way to much stress on me and it’s just getting to be to much.

Yeah, it’s fine to stop doing that, you know already that it really doesn’t work, it will take some time for them to adjust to being the adults rather than the kids, keep yourself busy while they do it.