i (20m) have raised my sister(14f) since she was born, but i don't live in the same house as her anymore so now i only provide emotional support / guidance / that kind of thing, but this context is important because it's not the kind of thing siblings typically talk about with each other. i would have preferred for her to have this conversation with our mother but it's out of the question here due to family stuff.
anyway a couple days ago i accidentally pressed on a snapchat notification on a phone she borrowed from me a while ago, and it opened up to an ongoing very sexual conversation with one of her classmates.
snapchat deletes messages over time, so i don't know how long this has been going on for, but they clearly have the intent to have intercourse soon, or already have.
there are two issues here for me. the minor one is that she is already in a relationship so this is cheating, but the major problem is that she doesn't have condoms or anything like that.
i'd prefer for her to not have sex at all at this age, but obviously i can't stop anything she does, so i need to have a talk with her. she's coming over to my house on sunday and i will bring it up with her then.
i want to talk to her about safe sex, consent, and the fact she should wait until she's older. i'm not sure that i will bring up the fact it is cheating because the main concern is pregnancy and i don't want to hurt her feelings more than this conversation already will. (and to an extent her personal relationships aren't my business)
but i don't know how to go about it tactfully. i've always wanted to respect her privacy and she is very emotionally sensitive so im worried about how much having something like this discovered will affect her? i am the only person in our family she talks about her feelings with.
all advice for the upcoming conversation is very appreciated, also anything else i might need to bring up?