Okay ladies I need some advice.

I have a little sister and her mom lets her do whatever the hell she wants. She's 12. She smokes marijuana with her friends she drinks alcohol with her friends and she talks to men that are almost in their 20s. Her mom has asked me for help with trying to discipline her and things like that but after i help her it goes back to the way it was. My sister walks all over her mom. Our dad died 8 years ago so there's no saying nothing to him. Ive tried and tried with my sister and her mom just contradicts what I say/do.. Im worried about my sister. Should i call cps or just leave it be?