Mother keeping sister's social security benefit money

My younger sister was emancipated at the end of 2010. Her and I both received social security payments because of my father who passed away in 1997. I am in college and no longer receive the benefits. However, my sister is now 18 and my mom has not been giving her her payments, and instead, using them on herself. How can my sister directly receive the payments into her account? When I was still getting the social security payments, they went directly into my bank account, but I don't remember how I made that arrangement.