SIL's boyfriend

Would you tell your husband and SIL that her boyfriend has touched you? When it first happened we were all in a bar and he came up behind me and sort of brushed up against my ass. At first I thought it was a mistake because it was really crowded but second time we had dinner at his house and he placed his hands under the dinner table and rubbed my left thigh until I jumped up. Last incident was over a year ago. I really want to tell my SIL because their talking about marriage but I'm worried it's too late now. I haven't seen them in a year as their overseas for work, but what if he continues to do this? I'm very skeptical to come between a woman and her man but feels like she needs to know the truth. If he's so brazen to do this, who knows what he's doing with other women? I'm just scared she won't believe me and it will cause a rift within the family. What would you ladies do?