I'm so sorry your mom did that to you, she really should've not said you are stupid, your sister is also doing the wrong thing. Also read my comment on My mom got in a Fight with Me. I notice that it's by you. Another thing, you are not a mistake you just don't know what to do. Your mom is probaly either jealous or stressed out in her life. Tell your grandma how you feel and tell your sister and your mom this "I'm sorry if I did anything wrong to you, or if you might have a hard life, but I'm sorry" or maybe something in the comment of My mom got in a Fight with Me.