My (20F) sister (27F) is having an affair with her close friend’s (40-smthn F) husband (40-smthn M) and I dont know what to do about it.

This, is obviously a throw away account. So let me start of by saying this. I love my sister to death. Growing up in a household with an abusive mother, we were always close and looking out for one another. But yesterday, me and my boyfriend were watching Netflix on my sisters old iPad that she left behind when she moved out. We’ll call my sister Stacy. While watching it, we see a notification pop up, and I realised Stacy forgot to log out of the insta on her iPad. The notification was from, lets call him Saul. The texts were questionable, but because Stacy and Saul are best-friends, me and my boyfriend didn’t think much about it. But later we changed our minds and decided to take a look. And I wish I hadn’t honestly. There were sexts, she send him videos of god knows what and worst part? They were having an affair. It wasn’t just a flirtatious friendship or sexting or just sex, not that its excusable, but something about having an extra marital love affair is just horrible. Now Saul has a family, he has a wife, we’ll call her Peach and a child (15M). Its horrible to know, that Peach and Stacy are also super close, and neighbours. When Stacy first moved in, Peach and Saul took care of her and bent over backwards to be there for her. I visited Stacy a few months back and Peach was nothing but sweet to me. It breaks my heart to see this happen to her My main problem, however, is that mu mother is extremely abusive and has mental issues, and Stacy is my only help in the whole country, every other family member of mine live abroad. I want to tell Peach but I also don’t want to be completely helpless in any possible situations. Also im not confronting Stacy because this isn’t the first time shes done something like this and im just done and disgusted. Please help