I would ONLY TELL IF I SAW IT - IF I HAD PROOF!!
When you assume - you make an A$$ out of you and me. No assumptions, no gut instinct, no preconceived notions...she obviously hasn't forgiven her BIL and doesn't trust him. Her sister has. It's her sister's life. NOT HERS.
If I knew for a fact that my BIL was cheating. I would tell him he has X amount of time to tell or I will. Same with my sister or brother. that's not cool. IF you aren't happy in your relationship - then YOU need to make the change...
Would I want to ruin my sister's happiness? No. I would NOT do it on an assumption. Too many people assume they know something as fact and they are wrong. But they go full speed ahead and yap away about things they believe are "fact" but are not. You don't ruin someone's life on assumption.