First of all, if you can stop a fight between your sister and yourself, why can't you stop one with your mom?
Secondly, when I have a fight with my parents, I find that it is best to just quietly explain that I understand that they are upset, and I'm sorry that I (or something that I did) was the cause. I then leave the room.
I had a lot of troubles with my mom when I first got interested in magic. Now I am much older and I have no need to share my spiritual choices with them. They have already expressed their feelings about it, and there is no need to discuss it further.