u/Amazing-Hotel3468

So I’m in kind of a weird situation and I don’t know what the right thing to do is.
My sister recently started dating this guy, and when she first told me about him, she said that he and our cousin had “seen each other briefly.” I didn’t think anything of it at the time.
Last night I was hanging out with my cousin, and she told me that they were actually more involved than that. It wasn’t serious or anything, but they did hook up more than once. She also specifically asked me not to tell my sister.
Now I feel really uncomfortable. I don’t know if my sister has the full picture, or if her boyfriend kind of downplayed it, or if this is just a difference in how people describe things.
I don’t want to create drama or betray my cousin’s trust, but at the same time I feel like my sister deserves to have accurate information, especially since it’s someone in our family.
For context, I don’t know him well enough to really have an opinion on him otherwise.
What would you do in this situation?
Would you stay out of it, encourage my cousin to tell her, or tell your sister yourself?
Also, personally, would you want to know if your partner had hooked up with one of your family members, or would you rather not know if it was in the past?