I'm going to tell my older sister about my younger sister's marijuana use
Do you feel that's necessary? Why are you going to tell your older sister about your younger sister's marijuana use?
I want her to speak with my sister and get her to stop using it.
I'm sure you're very concerned about your younger sister's drug use. It's good to tell your older sister so that the two of you can deal with your younger sister's problem. Try to get her to stop her drug use now before it becomes a serious problem.
I'm going to do it because it's not my problem.
Even though it's not your problem, it would be great if you and your older sister could try tackling it together. Maybe if your other sister sees that you both care she'll be more likely to try to stop.