I’d encourage you to seek couple counseling together to work on a resolution. I don’t think the decision you make jointly matter so much as the fact that you’ll need to be on the same page.
You future is with your fiancé and while cutting off from your family might be the right answer for you as a couple, you may want to explore in therapy the alternative of you seeing your mother and brother on occasions by yourself.
There isn’t a ready-made answer for this sort of situation. But each of you have chosen each other because of conditions that prevailed in your families of origin that you didn’t have control over. The two of you working together to experiment with a solution is already a step toward healing the dynamics inflicted by your families.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral