Venting.😤 Advice? Comments?

My future sister-in-law drives me crazy! She's been spoiled her entire life; she's ALWAYS getting whatever she wants. She's 20 and has a baby that will be one in July. Whenever she doesn't get what she wants, she threatens to kill herself. I honestly want to beat the shit out of her and tell her to grow the fuck up. My fiancés parents take care of his sister's baby while she goes out and parties and does drugs and sleeps around. His sister has always lived at home and has done nothing for herself to improve her life. She didn't graduate high school, she has never had a job or had to work for anything in her life, she doesn't have a license and because she's so spoiled, doesn't know how to cook or clean or really do anything for herself. Her baby cries for her and will reach for her and she completely ignores her daughter. About a week ago, she tried to kill herself by taking a bunch of her sleeping pills. I would normally feel really bad about it and try to help her but I am FURIOUS! Her parents stick up for her and say she doesn't have purpose and I want to fucking scream! She has a fucking daughter that loves her and needs her; yet, she's so fucking selfish and only thinks of herself. I'm currently over 35 weeks pregnant and I had to take care of her daughter while she was in the hospital. My fiancé said that he and I might have to adopt his niece soon because his mom is moving out of state with his sister and niece and his sister won't take care of her own baby. Of course I would take care of family but wtf am I supposed to do?! I'm nearly 19 and am having my first child soon. My fiancé and I can barely survive on his pay and I've been in too much pain to work. How the hell am I supposed to raise two babies when he works 40-50+ hours a week and we don't have enough money? On top of that, I don't want to raise his sisters daughter before I raise my own. Am I being selfish?? I just want her to grow the fuck up and take care of her daughter so I can take care of mine.😡😤