I need advice for my bonus girl (step daughter)

Hey yalll so a mama needs some help. I have two other children a 1 year old son (which isn’t the issue) and then my 3 year old daughter, and my 7 year old bonus girl. My step daughter really dislikes my 3 year old. So a little background on my 7 year old bonus girl is a only child at her moms. Her mom doesn’t discipline her at all, not even time outs or corner when she does something wrong. Just as a example, I’ve seen her paint on white walls with blue acrylic paint, and her mom was laughing and recording her doing it. So a little info on my 3 year old, we are getting her evaluated for Autism and ADHD but she acts out. Hits, bites, and says very hurtful things to everyone in the home, when she does this so does get disciplined for this. My fiancé and I normally do 50/50 with his daughter but here recently she has been refusing to come over saying she never wants to come over again because she gets in trouble here and because her sister is mean to her. My bonus daughter is terrible with sharing (could be because she doesn’t have to share at her moms). We haven’t seen her in 3 weeks and we miss her so much but we aren’t even looking forward to getting her because we know she doesn’t want to be here. We just don’t know what to do in this situation. We’ve tried everything and are so lost at this point. We can’t just shower her with special treatment and let her have no discipline when she’s with us. She lies often to get my daughter in trouble or will do something that will be an accident and my daughter will get hurt and even with it being a accident she will still lie and blame it on my daughter. We’ve seen my step daughter do things to purposely make my daughter angry and my daughter lashes out because she intentionally made her mad. I just don’t know what to do anymore y’all. We talked to her mom about everything going on and of course she just thinks my daughter is the issue and refuses to see any wrong in what her daughters doing. My stepdaughter just says everything that my daughter does wrong to her mom and never says that she does things wrong over here too. It’s getting to the point where we are just going to have to go to court to get a new custody agreement. I could write a book for y’all about all of this but any input is appreciated I just want my stepdaughter to be happy and this is not the relationship I wanted with her I’m just heartbroken and don’t know what to do anymore 🥺