What should I say to my brother in law about his cheating on my sister?

Focus on helping your sister. Let Karma work out the rest.

You were right when you said it wasn't your business. It isn't. Stay out of it and let your sister handle her own affairs. BUT! That's not to say you should not be available when your sister needs you. And boy, will she be needing you now. The thing about butt-f**ks like her ex is that nothing you can say will change them. They don't care a whit about what they do, why they do it, or whom it hurts. So concentrate on helping with the healing, not the revenge. Besides, she can get him with the court ordered child support she had better go for. But make it clear, you aren't her Fairy Godmother, ready to make everything all wonderful. You are support, here to help her stand on her own two feet until she can support herself. Good Luck, and have plenty of Kleenex on hand. You're going to need it.

This is between your sister and him stay out of it. Do your best to help your sister stay sane and get through this as fast as possible. If she was the homemaker and he left. She can take him to the cleaners. Let that be your statement to him. Anything more will only hurt your sister and her case against him.

Don't bother with him go after the guy that is the dad to the daughter and son. He needs to pay child support. Then she would have some income.

Don't waste your time being clever with your brother-in-law, use that energy to help your sister zap him where it hurts, his wallet. Go with your sister to legal aid and have his wages garnisheed for child and spousal support. Then he will know the depth of your anger for him.

Your energy should be helping your sister.

Your brother in law is not blood. Why waste your time on him, when he chooses to play your sister like that.

Hes really not worth your effort even thinking about him is it...........

This is between your sister and her husband. Do not stir up the pot further for your selfish petty need. You will only hurt your sister.

If you want to "stick up" for your sister, then help her get a job.

This is their problem. I know it is your sister and all but it is not your place to get involved.

Just let him know YOU KNOW...subtle- like...he'll do the rest.