I have experience with meth. I used to be a user although I never did it a lot it completely screwed up my life... Very recently I quit so I can give advice.
Meth is a terrible drug. Probably one of the worst out there if not the worst in how it changes people. It can completely change a person into someone who they are not so please keep that in mind. Your sister has more than likely reached the stage where she is not even herself anymore.
You have to realize that until she wants to change there is nothing you are going to be able to do for her. You can't force a person to quit she has to want to do it for herself especially with a drug like meth. So until she reaches that stage you have to let her go.
That said meth isn't physically addicting so when she decides to quit she can do it with help. It's not like heroin where there is a strong physical withdraw which scares a lot of addicts from trying to quit even when they want to. It's all mental and the benefits of quitting meth are so huge it makes it very doable when compared to other hard drugs.
Not that it is easy especially if a user has been using for a long time. I hear with longtime users depression is going to happen sometimes for months due to a low level of dopamine but when she reaches that point when she is ready to quit get her professional help ASAP and she should be able to beat it.
Until she reaches that point I'd stay away. Talk to her but don't mention quitting just talk to her like you would anyone else. When she is ready she will let you know. Like I said you can't force someone to quit a drug like meth they have to want to do it on their own and frankly they are going to have to be determined because although I'd say it is easier than heroin it certainly isn't easy.
Hope your sister can beat this!