I would talk with my sister: I would tell her that what she is doing is not right, that she shouldthink things through before doing them, otherwise she would regret it. If she really wants to meet a new boy, her current boyfriend will understand, and it will be better if he finds out from her than from a third person.
I would do everything possible to make her think about it, to tell her boyfriend the truth, or not to meet another boy. In the case that she does not accept any of these options, I will have to talk with my brother-in-law. I do not like doing this to my sister, but I also have a lot of appreciation to him, I do not want him to suffer for her, and I should tell him what is happening.