Problems with my sister..please help? emergency

alright im just going to cut right to the point
people tell me they saw my sister at a party, and they saw her mad drunk and kissin mad boys
i know she drinks
and iknow shes like a really popular person in her school
i know she gets drunk with her friends and does stuff
and im really ashamed of it and embarassed of it and i dont like talking about it
but i need help
alot.
some girls telling me theres rumors that she gives head and does mad stuff with guys and always goes to parties and gets drunk
idk whats true or not, i know a good handful of them are
what do i do though?
do i ask her if she drinks?
do i tell my mom but then tell her not to mention it to my sister?
do i jsut keep my mouth shut?
please, please, help me.
i dont wanna tell on her in a way because like shes been being so nice to me, i dont want that all to go away, because shes never like this with me and i dont want her to stop like helping me with stuff and telling me things.
just help me..please.