Should I tell?

My sister told me, my other sisters and my mom that she’s been cheating on her husband with a random man from Instagram. while her kids are in the house!!! I feel that this puts them in danger.... she is also not using protection with this man, which could put her husband at risk for STDs. I’ve told my sister my concerns and tried to give her advice without being judgmental and she says she doesn’t care if she’s hurting her husband, the marriage etc BUT she also said she has no intention of leaving (or telling) her husband because that would mean she would need to get a job....... Obviously, there’s something really wrong with her.. I’m wondering if I should tell her husband.. on one hand I feel like it’s not my place and I should just let it play out.. but on the other, I would want someone to tell me if I were being cheated on and I know I’ll hate myself if something happens to the kids... WHAT SHOULD I DO?!?!

If you think I should leave it alone, how can I make sure the kids are safe?

If you think I should tell him please give me advice on how to tell him.