Soon to be sister in law help.

PeacefulMama • Trying to convince #5 2016💜 2018💙 2020💙 2022💜 TTC🤞🏼
(I hope this is an okay place to post this)
My soon to be sister in law is 17, I'm 21 and pregnant with her niece. She's a junior in high school. Well her and I don't seem to like each other. The thing is she acts fake to my face. I can't stand it. She acts like everything is good between us. My friend's little sister has a class with my soon to be SIL and heard her talking crap about me. She blames me for her brother never coming home. The thing is he goes to college and works a lot. I barley see him. She called me "that girl" and when someone brought the baby up she freaked out and told them to shut up. I told my fiancée to talk to her and explain that it's not my fault he never comes around. He did and she said she understands. I don't know why she cares about him coming around. She doesn't make an effort to see him when he's there. He'll try to get her to play basketball and she whines and says she has stuff to do or she's tired. When he proposed to me all she said was "well congrats." I know this sounds petty, but she hasn't liked a single picture about the baby that's on Facebook and Instagram, not even the ultrasound pictures. But she'll like all the other bother's girlfriend's pictures that are on Instagram. 
I just don't know what to do because I know I'm going to go crazy when this baby is here and she wants to all of a sudden take interest in the baby. Any advice? Am I just being dumb and need to let it go or do I have a point?