I don't know her motivations, but I do know that what you described exactly matches my mom. I can't say for certain she has a drug problem, but I can say for certain my mom has a severe drug problem (I'm amazed she's still alive), so it seems likely (to me).
I don't think it's possible to reason with her to get her to straighten out; she either will or she won't, and it's all up to her. The only advice I can give is to get those kids out of there and into a healthy environment, or she will drag them down with her.
Everytime I tried to achieve anything it was like getting the rug pulled out from under me, ending up flat on my ass again; those kids need a strong foundation if they are to build a life for themselfs, they need someone they can rely on.
