Sister allowed her bf to threaten our little brother…

Not sure what to do and I could really use advice. This is very long, but it explains a lot of key points to get the best feedback.

A few years ago, my half sister blasted on her social media that her stepfather (we have the same dad but she was raised by her mom and her husband) molested her from 12-18 years old. She posted a vague screenshot between them that nobody was able to understand. It does look a little suspicious, but from what I remembered of her growing up (I’m older), she never listened to our mother and got whatever she want and got away with everything with her stepdad. She was attached to him. They’d go everywhere together. Not coerced or anything. She ran that side of the house and he’d let her and my mom worked too much to enforce much.

My sister divided the whole family because she got mad at mom for not leaving her husband of 25 years. She doesn’t believe her. My sister is known to be a pathological liar, so she’s making it hard to believe even serious things and it wouldn’t surprise us if she was because the reason she got mad at him is because our mom finally made him cut her off. As in stop paying for her. She was 26 at the time and still on the family phone bill that they paid and paying for her car payments and insurance. And it’s basically mom paying it all because she’s been the one working.

My sister always blasts all over FB that her parents are horrible and weren’t there for her and everything. Our little brother (21yr old) who listened to mom growing up and respected the rules, saw it all. Calls her on her shit talking all the time and they fight, but our brother is tired of her. She’s a leech. My brother is a great young man. Life was not hard. My sister was a delinquent. Sneaking boys in all the time and such that they had to nail her window shut growing up.

After the accusation a few years ago, I was there for my sister. She had me convinced that she never went over to the house anymore and she didn’t talk to her stepdad at all. Claims she blocked them. Cool.

My bf and I go to visit the family for the holidays this past week and came to realize quickly that it was ALL a lie. She dropped those kids off every second she had. She left them at mom’s with us 5pm on Christmas eve and came back shitfaced drunk around 1am Christmas Day to pick the kids up. Luckily with a DD, but she almost dropped her newborn. Mom was LIVID.

My sister dates someone new every month or so and is the type to post her new relationship on FB immediately. It’s a joke but I never say anything because she actually knows how it looks. She doesn’t care. She always says “I don’t give a fuck” if anyone says anything against what she’s saying or doing.

As of two months ago, she’s dating a new guy that she moved in with and is part of a biker club. The first guy that has ever taken care of her and provides, but their lifestyle around the club has her carrying around a pistol like she’s part of a Sons of Anarchy.

Now that there’s some back story, now to the current issue. My bf and I are about to leave Tuesday morning and everyone’s at the house to say goodbye to me one last moment. My sister said something disrespectful that I didn’t catch, but our brother called her out quick. I didn’t realize what was happening until I heard him ask “why talk shit about mom and dad and ask for loans and someone to co-sign?” We found out our sister asked mom to co-sign on a car last year. Mom did it so she could have a car to take care of the babies. Our sister didn’t make any payments and it got repossessed. So now, a week ago, she’s asking for a loan from mom. Asks her to co-sign again…then asks her stepdad for a loan and co-sign…for her boyfriend’s motorcycle. A $23,000 Harley as a Christmas gift!!!

She was shocked we knew and her response took a second and then said it was owed to her for all the shit she had been through. She started mentioning times in the past that both my brother and mom denied ever happening claiming she was locked out and that she worried for her brother and always took care of him and protected him. I never saw this either. I saw my sister being completely wild and disregarded all rules growing up.

She ran out of the house mad and told us all to fuck off. Then ran back inside to yell at mom saying she’s not ever bringing the kids back there again. She does that because she knows it breaks our mom’s heart. She cares for the babies and takes care of them most of the time.

Within minutes, our brother gets a text from our sister’s biker bf talking about “…we need to talk…come meet me…I’ll find you and run up on you with a pistol…watch your back…”

We stuck around because we weren’t allowing that to happen. We were supposed to drive back home but we waited because my bf and I knew we’d have to be the ones to call the cops if her biker club showed up to touch a finger on our brother.

I didn’t have anything to say to her. I guess she told her bf that she’s mad at me for not defending her and suggesting to call the cops. Started talking shit about us and we didn’t even do anything.

My bf handles these things differently and said to call CPS on her and to cut her off. It won’t matter if I cut her off. I believe our mom is too weak and kind hearted to cut her off because she doesn’t want anything bad to happen to the kids.

I don’t have much to say to my sister but she’s not talking to me right now. I can’t believe she’d allow her bf to threaten our brother like that. He shouldn’t be involved at all aside from a listening ear since she’s mad, but to take it upon himself to send a threat to his phone, which she gave him our brother’s phone number to do so, is unacceptable! Been waiting for her to ask me herself why I didn’t defend her rather than telling everyone else that she’s mad at me and thinks I’m rude for not defending her.

She needs to know that if she wants people to believe her and sympathize, she needs to cut the family off. Stop coming around and asking for favors and handouts while talking shit about them and claiming you’re not involved with them because of everything, when she actually texts her stepdad every single day!

What should I do? Do I cut her off? Even if I did, what do I do if our mom can’t and my sister continues to manipulate the family? I understand they’re enabling, but idk how to go from here. Do I call CPS? Do I just leave it to the family to deal with over and over? What do I say to my sister eventually?

Any advice is welcome. I love my sister but she’s a mess right now and she’s going to continue to tear the family down and risk her kids if she doesn’t get her shit together soon.