Drugs :(

I wasnt sure where to post this, but ive juat discovered my little sister (whos 15) is doing drugs!

I dont live with my family any more, as i have moved out, and live quite far away now, and ive found out over social media.

I know i must tell my mum, but i dont know how, or when... this news has made me feel really sad and i dont know how to deal with it... :(

Does anyone have any advice? Thanks guys, any advice or tips will be grately appriciated right now :)