What to do

I am 28 weeks and 1 day and my mom has custody of my nephew who is 3 years old and he has ADHD. For the past month I am always the one that is having to watch him. She don't even ask if I mind watching him. She just says I will be back and walks out. My sister( nephews mom) is staying with me and my boyfriend since August. And she has done failed one drug test for cocaine. She is with a guy that broke into my house and stolen my Xbox and some other thing and she don't care. She can still see her son at anytime she just has to be watched. But all she does is sit around and always on her phone. She has not got her son any thing. It is me and my mom getting him everything he needs. But my mom is still taking up for her. And is taking her back and fourth to work. And I am to the point where I am about to go off. All she does it sleep all day. She don't help with cleaning or anything. And I am having to watch her son. But I can not get my mom to see that my sister is using her.. What to do