My best friend is being mean?

My friend (we have been best friends for 7 years) keeps bringing me down. I know for a fact she has a really bad home life, her parents are mean and make her take care of her baby brother and sister, she is only 12, and she has way too responsibilities. However, shes making my self esteem bad because she keeps brining me down to make herself look good. She also never says thanks to my mom when she takes us places her mom would never dare take her because shes too busy smoking. I don't know what to do, I've tried to tell her in a nice way about the not saying "thanks" part and she didn't know what I was talking about. Please help!