URGENT!!! Should I tell someone about my twin sisters problem?

So im 16 and i have a twin sister. Today in cl she was upset and was saying that shes been having dreams about killing herself and stuff like that. Also a few weeks ago she told me she always felt like she wouldnt live past the age of 18. She's not different, shes normal and we've had our family problems in the past. I don't know if I should go tell a school counselor or something cuz then if the school finds out, theyll think shes crazy and make a big deal about it. We got to a therapist but the next time we see her isnt for another week, so i feel like i should tell someone, cuz god for bid she does something(which i dont think she will) but i was the only person who knew. Should I go tell a school counselor tomorrow?