My boyfriend is being bullied by his own family

My boyfriend has a brat of a younger sister and tho she is nice to me and I have never had a personal issue with her she bullies her older brother (my boyfriend) who is too nice and hates confrontation to say anything back. She always gets what she wants and will complain and throw a fit if someone tells her no and will cry until she hears yes and it is always at the expense of my boyfriend. She is the same age as I and he is 3 years older (all of us in our 20s) and she acts like a 12 year old and her parents don’t do anything to stop it. Its not my place to say anything it is not my family and we have only been dating for a year and a half but I losing my mind watching this girl disrespect her entire family especially her brother and her parents not saying anything to her or if they do they feel bad after and let her have her way again.

Has anyone been in this situation before? How did you handle it because my blood is boiling with what has unfolded in the last week watching her bully him until he gave up something he had that she wanted and their parents just sat and didnt say anything. I am beyond angry watching someone I love be emotionally and verbally abused just so a brat can get her way all the time but not being able to say anything. 😡😡

All that consoles me is the hope that she is going to have a tough road ahead when someone finally stands up to her. She always pulls the mental health card too that “all of this is so bad for my mental health” like bitch you’re a bully you’re bad for everyones mental health.

Update** they both still live at home this is why the parents are involved. I agree they are both grown adults and I know with my siblings we argue but we can always sort it out ourselves no parents involved. In this situation the bullying in ongoing in the parents house while the parents sit and listen to her verbally abuse all 3 of them and if anyone tell her to grow up and stop she throws a fit until she gets her way. He hates what she does to him and has lived away from home for the last 5 years but has had to move back due to the pandemic and thus has no way out but to put up with it and accommodate anything she wants otherwise she makes his life a living hell.