As much as i LOVE pot...
It sounds like she's just a horrible person in general who happens to smoke pot. That's like saying driving cars is bad because sometimes bad people drive cars... it's just displacing the blame onto something tangible.
Blaming the pot instead of her mother's poor parenting in general is a bit of putting the cart before the horse.
Sounds like she's been shitty for a long time. Shitty, abusive parents are shitty, abusive parents- no matter what they drink, smoke, etc.
If she were wasting $300 on shoes every other week, no one would blame the shoes. Everyone wants a scapegoat rather than saying someone is a bad parent.
la petite moi- you're wonderful, you're great, you're fun, we love ya... but go get a fuckin' job already.
If nothing else, do it so that you can help your sister. i'm sure she needs it.
It's easy to demonize pot, and people don't like saying other people are bad parents. Well, maybe she is. We should face that. Now, help your sister.
Good luck.