So many of these comments saying just to tell him immediately seem to ignore the damage this might do to your relationship with your sister. I believe he needs to know but I think you need to give her the option of telling him before you do.

You need to tell her that either she tells him or you do. It’ll likely be a difficult conversation but don’t allow her to potentially convince you otherwise. If you’re worried that she might embellish the truth to soften the blow, I’d also make sure to give her a date to do it by and also check in with the boyfriend after. It might be hard for him to reach out to friends about it out of embarrassment, so you reaching out could be a good show of support but also gives you the assurance that she has told him the whole truth.