Good for you.
You are doing the right thing by telling your mom, and maybe your sister will grow up after your mom have a talk with her.
I wish that I told my parents about my 2 slutty pothead sisters when they started doing drugs.
One sister didn't finish high school and had her first kid at 19 year old (total 2 kids).
The other didn't finish 9th grade and had her first kid at 18 year old (total 3 kids with 2 different dead beat drug dealers). She has been on & off the street for over 15 years, and has been in the hospital countless times for alcohol & drugs poisoning. She started out with pot, and now ended up doing anything that she can get her hands on (including opium, crack, and crystal meth).
The rest of my 3 siblings & I doesn't smoke or drink, and we all have post secondary education. One have PHD, 2 have masters, and one have 2 bachelors.
The moral of the story is that, the very least drugs does is inhibit your ambitions.