Assalamu Alaikum - My sister-in-law is betraying my brother and now her sisters are involved

Assalamu Alaikum. I’m a 21-year-old woman living in my parents’ home with my sister-in-law (25F) and her three children. She married my brother when she was 17 and he was 20. He’s been away for a few years for reasons I can’t share, and it was agreed before he left. We each have our own rooms and some privacy here. Lately I’ve noticed the kids aren’t being cared for properly - they fall asleep anywhere, don’t brush their teeth, and they’re watching things online that aren’t appropriate. I keep stepping in because they’ve begun copying bad language and behavior. A few days ago I saw suspicious calls and messages on her phone. I know I shouldn’t have looked, but I had a strong bad feeling. I found voice calls between her and another man. I spoke to her privately and calmly, told her if she is unhappy she should seek a proper divorce with dignity instead of betraying my brother, and I said it as a concerned Muslim sister - not just as his sibling. She didn’t deny it; she just went quiet and has been distant since. Before I confronted her, I told her older sister (19F) who’s lived with us a long time. She’s known my sister-in-law and my brother since she was young and sees my brother as a father figure because her own dad was absent. She was heartbroken and angry and confronted the younger sister (17F), accusing her of introducing my sister-in-law to that man or covering for her. The younger girl had let the older sister use her pictures and seemed to know what was happening. When challenged, the 17-year-old became defensive and said she’d done nothing wrong. Things got heated and nearly turned physical, and we had to separate them. Now that younger sister has come to live with us, and honestly she’s a bad influence - she dresses and acts older than she is, brings inappropriate energy into the house, and the kids see all of it. I’m trying to keep peace and protect the children, but the home feels like it’s falling apart. I don’t want to needlessly shame anyone, but I can’t stay silent when something is clearly wrong - both morally and in Islam. Should I tell my brother what I found out? Should I speak to our parents about the turmoil and the children’s wellbeing? Or should I stay quiet for now and focus only on protecting the kids? I’m exhausted from trying to keep everything together. JazakAllah khair for any advice.