Need advice on BIL not disciplining his child when they're violent towards mine

My husband's brother's daughter (5 yo) has shown some very concerning behavior towards my daughter (1 yo) but what's most concerning is how my BIL doesn't do anything about his daughters misbehavior.

This little girl, we'll call her Ann, has always had a pretty...strong personality. It had always been evident to me that she was babied by her parents a lot and didn't really have expectations to behave well. Basically she's a brat with a capital B. Shes bossy, tells adults what to do, throws tantrums, and the problem I've been having with her is her jealousy.

Just recently we had daughters first birthday party, I invited the people in the family with kids and the grandparents. I thought since now my daughter could walk and really play she'd have more fun with her cousins. Well, Ann showed up in a very bad and jealous mood. She wanted to open the presents, she wanted to blow out the candles, she wanted to eat daughters smash cake, she wanted her toys. Every time I would turn my back and try to talk to an adult family member I would soon hear my daughter screaming and crying because Ann threw a toy at her, pushed her down, at one point when my mom was with my daughter pushing her in a wagon Ann decided to shove the wagon as hard as she could causing my daughter to fall out onto her back. I didn't even know that happened because my mom comforted her before I heard any cries and told me about it afterwards. If she had told me I probably would've lost it with BIL right there. My mom said all BIL said to her was "I know you're feeling jealous, when we're done here we'll go to the waterpark and daddy will buy you a toy"

I don't know what to do about it, I told my husband something has to give. It sucks because I really do love my BIL, SIL and niece. We already have problems with some of husband's family and I don't want to have to cut off all of them. Any advice?