Help 😑

My boyfriend has a little sister (14) and today in school I found out from mutual friends that she had sex with a (16) year old that's left school,I had people coming up to me and asking if I'm going to tell me boyfriend and I was stressed as what to do as I'm really close to her and she has trusted me with a few things in the past and my boyfriend is really protective over her and has some serious anger issues

I did end up telling him but now she is in serious trouble and I think she may be upset with me but I haven't told her yet that it came from me?

Do you think I done the right thing?