Boflo, what is happening between your sister and her husband is none of your business. as much as you feel to be part of their family, the only person you are related to is your sister. DON'T tell your brother-in-law anything. On the other hand, because you were raised together and share a lot in terms of education, morality, customs, core values and traditions, I think you should talk to your sister. Don't confront her harshly, don't be mad at her, don't be a moralist.
Just tell her that you know what she is doing and (in your humble opinion, for what it's worth) you think that she is putting her marriage in danger. remind her of how good (you think) her husband is to her and to their kids, and also mention the financial assistance he hs been providing the extended family (if it is more than you). then let her decide what to do.
If she does not change her behavior, and if you are afraid that once busted her husband might think you supported her, then just stay away from her to manifest your disapproval.
Narudia tena: what is happening between her and her husband is NOT your business, all you can do is provide humble advices to your sister.