It is a tough situation because you love your mother and want to honor her wishes but I think you should stand firm on not being there when your brother and sister-in-law are there. Mothers want there kids to get along but when they are adults, we just can't fix it anymore. I wasn't even able to fix it when they were little kids--fights have to run their course. If you cave you are allowing them to continue to act badly. You don't need this. It is uncomfortable when we stand up for ourselves but in the long run it makes us feel great. It is taking our power back. As my sister likes to say, "Girl power!!"