Vent or do something about it

Idk what this is. I think I just need to put it out there somewhere. My sister is married and has two kids. They’ve been together over six years now. Met online, married six months in, obviously had ups and downs. This latest thing is just getting to me though.

She’s my sister I truly care about her but I feel like she isn’t making the best decisions. Some background to include, she’s cheated through out the relationship, not sure about him. They both have high sex drives and body counts. This doesn’t matter to me but my sister is basically cheating on her husband and that’s her excuse.

The night before they married at the courthouse, she cheated. She’s cheated with several men through the six years, more recently with her longtime friend. A man she met when she was underage. She was 14 when she met him and he was probably around 25, not sure. They’ve always had a thing and will continue. A few years ago my sister felt like she was breaking and was serious about leaving her husband. Literally set it all up with papers and everything. Toward the end she went to see the fwb and cheated once again. I tried my best to support her because I really believed she’d be leaving her husband. A couple months later she bails on the plan because her husband promised to changed. Now again she feels like she’s in the same place and wants to leave.

Her husband is away training for border patrol, a very demanding job, and she’s at her in laws with her kids. The other day she told me she can’t go long without having sex and she’s wishing her and the husband established being open and sleeping with other people. She’s visiting in my state right now and I’m pretty sure she left one morning to go meet another older man she met when she was underage. She’s been messaging and sending packages back and forth. There was even an I love you text. She had me watch her kids when she went to “get her tires changed”. I bit my tongue and left it at that. I absolutely know something is up though. She’s basically admitting she doesn’t want to be with her husband and wants to sleep around.

I’m incredibly understanding and I’ll try to support her if she ends up a single mom again but idk If I can if it’s going down this way.

What bothers me though is that she’s not even attempting to communicate with her husband. She said she simply wouldn’t be able to do she doesn’t. Why be with someone you can’t talk to?? She’s blatantly jeopardizing her and her childrens relationship with him for a few hookups. If she wants to be a free woman and sleep with whoever I couldn’t give a feck less but I can’t support someone cheating like that. I’ve been on that end of things. I take this personal even though i know it’s none of my business. He’s not a terrible guy, they both have their issues and I see both sides of things.

So would you say anything to anyone or just keep this to yourself and stay out of it??