How do I contest my brother in law inheriting my mother's ira.

I believe my sister used undue influence for many years to change my moms ira beneficiary to my sister 100%. Previously my father, sister and myself had always been named. Also in the notebook she was listing assets in just prior to passing mom named all 3 as beneficiarys. Then my sister died suddenly a few months after mom. Sister lived in parents home with husband and 14 year old child since she got married, about 20 years. Mom always paid for everything in th household. Sister was supposed to set up meeting with lawyer for estate planning but it somehow was never done. Sister took over everything after mom passed because dad has never dealt with any type of finances or household responsibilities and his health was declining from dementia. Prior to passing sister talked about repairs she was going to make on the house. After sister died brother in law suddenly moves a week later. Now he will get all of my moms retirement instead of my dad and me which is what mom wanted