Should I tell my mom? TR*

A few months My cousin told me and that my Sister's boyfriend beat her up. I wasn't sure what to do. A lot of tough changes have been happening in my life , so I selfishly put this to a side. I started a new job and a coworker of mine was talking about her boyfriend and mentioned that, her boyfriend's cousin (who lives next door) has a boyfriend that beats her up and how she wanted to help her but she wasn't sure how to. After some questions, we realized that her boyfriend was my cousin and his cousin is my sister. He is hurting her and I'm not sure if i should tell my mom. They are living together with my two little nephews (not his). So im worried. My sister is 25 and I am 27. My nephews are 5 and 6.