"snitches are the lowest form of scum. maybe some day youll learn how the real world works kid"
his sister is a 14 year old girl with a group of friends who have stolen and drank alcohol obviously past the point of "just trying it". you're a moron if you think she's going to drink responsibly or not do anything stupid when she's drunk. (you're also a moron if you think they aren't stealing alcohol exclusively to get drunk)
unfortunately there isn't much OP can do to that won't at least temporarily ruining his relationship with his sister (setting his parents up to "find" the alcohol isn't going to change anything because she has friends who she can go to to get more/drink with)
EDIT:
If she only did it once or twice, it's probably not going to develop into anything truly bad.
If she's done it frequently, then maybe you should tell your parents. (Do not call the police.)
this is fairly reasonable