My stepdad is committing social security fraud. What should I do?

I just found out that my stepdad has been drawing additional social security on my sixteen-year-old brother who has been living with me for almost a year. My mother and stepfather pay me and my husband 300 a month for his room and board. This is well below minimum child support requirements. We all live in a 24 x 24 house. I’m trying to decide whether to turn him in because my house is so small and I’m afraid DHS will say it’s not fit and put my brother in foster care. What should I do?