What to do with a 15 year old girl having sex?

so my sister has came to live with me, now i have to go into the mom role ans sister role. she asked to if she go babysit and the next day i find out her boyfriend and her are having sex. she also mentioned that the condom had broke.. im not sure what to do or how to act...i told her she talk to her mom because she deserves to know but i also don't wanna ruin our relationship by telling if she doesn't..help!!