Hi, I can only imagine what your going through. It sounds like she may "acting" out maybe due to the situation she is in or maybe she wants some attention. May I suggest that you spend one and one time with her. I mean not even the baby with you. Take her somewhere she would enjoy but aomething that you both can interact with doing. Like to a museum she likes, aquarium, park, pottery class, or bake cookies but just you and her. Give her your undivided attention. Try not to yell or discipline her. If she doesn't cooperate just leave, try explaining to her that you will leave but u must stick with it or she will not take u seriously. Hopefully that would be a stepping stone to creating a realtionship with her. Do this frequently maybe 1 a week and also daily try to dedicate some alone time like 10-15 minutes of undivided attention with each child or if not daily a couple times in the week. This should make a difference.
Another thing you can try as u build a relashionship with the kids is create a chart based off of what you expect from them in a day ( brush their teeth, put away toys, no hitting, helping out) put a sticker or a check on each item each day and if they get. Certain number they can choose a paper from the jar. You can fill the jar with anythjng that u like, go to the movies, rent a movie, go to the park, one on one time with u, one and one time with dad and things like that.
Also, they need consequences to their behavior. Take away a loved toy, when they are on their beat behavior they can get it back.
If this doesnt work try speaking to a child psycologist. It is not a bad thing if you get professional help. I did when I was struggling with my son.
I hope this works out for u and I wish you the best.