I am 18 years old and I am wanting to get custody of my sister who is 2 years old my mom has a lot of mental problems and has very poor parenting she also smokes weed in front my 2 year old sister where she is getting exsposted I'm in the best shap for my sister and my mom thretend to him me infrount of her for telling how I think she should do good paringting she has a lesbien girlfriend who is very abusive to my mom and sister and my mom is very abusive to her and not in the right mind setting how do I get custody of my sister?

You would have to prove to a court that your mother is unfit and abusing your sister and that you can properly care for her. The first step might be to report the situation to family services telling them you are willing to foster her.