SO what do ya think about this messed up situation?

My bf and i are stayin at his parents temporarily cuz he lost his job. We are both 20 yrs old, and i am nine months pregnant. His sister keeps leavin her daughter at the house with me w/o askin so she can go off and do drugs and F*** her bf. She is older then us she is 21. My bf has repeatedly asked her to stop doin that but it has not stopped. Now she is doing drugs in the room next to ours. Plus her daughter is always climbing on me and sleeping with me and my bf becuz the mother will make her 5 yr old daughter sleep on the floor. Her daughter also told me she wants me to be her mom and is constantly wanting me to give her a bath, make her dinner, go everywhere my bf and i go. Plus she is always crawling on me and the other day she smacked my stomach. She always is wanting to use my unborn sons blankets,pacifiers,crib,toys etc. I love this little girl but it bothers me that i am seriously having to take on the role of mommy to her. I am so nerve racked about my own coming. Any advice would be greatly appreciated.