Grandmother not helping

I recently moved in with my grandmother and niece but my nieces behavior is out of control , she was behaving bad so i was lecturing her , then she threw pencils at me so I took away the tv , she also grabbed my water bottle and tried to put it in the trash etc. When her step father came to pick her up I told him all about what happen and she started to listen when he was lecturing /giving advice but the first thing my grandmother said was “well she provoked her” I’m obviously offended because why would you justify this child’s behavior. I was disciplining her and you did nothing/ said nothing while she was misbehaving. Idk what to do ... my family is so dysfunctional on top of that I help my grandmother with so many things and yesterday she said you dont help at all / what you do is not enough. She doesn’t see how much effort I put and never thanks me or supports me in anything in life.