First of all how much of her drug use is documented? Has she been arrested,in rehab, and/or did the children test positive at birth for drug use? Getting full custody can be an extremely hard fight, especially when some states are very pro-mom regardless of what her lifestyle is. Does she have any fixed address at all?
The first thing that you need to do is to build your case up. Keep phone records, as well as a record of when she visits with the kids, and for how long. You will have to be able to prove to the courts that she is without a doubt, unstable, and that can be a very hard thing to do. If she has yet to be in rehab, perhaps your husband could suggest to her that she go.
What you can also do, instead of going for full custody, is ask that her visits be supervised. You will still have to prove that she is unstable, but it is often easier than obtaining full custody. She will have to set her visits up through a court appointed supervisor, and pay for each visit that has to be supervised. This 'system' tends to work well with weeding out drg addicted parents, as they would sadly rather spend money on drugs than visits with kids. Many times parents make 2-3 visits and fall off the face of the planet though, so be prepared to answer some confused and sad children.....at least a little more than you already have to.
It truly sounds as though you want what is best for the children, and that is wonderful! Many parents get caught up in the 'who is the better parent' game, and forget that it is about the kids, not the adults. Start gathering your evidence, offer the mom some minimal help, so that when you go to court you can say that you have tried everything, yet the situation continues to remain unstable. You can even try going to court requesting supervised visitation once she completes a drug rehab program, and takes random drug tests, OR full custody if she refuses rehab. That way it is more her choice, and if she really is an addict, she isn't going to want rehab, and if she does choose rehab, then she has a good chance of getting herself cleaned up and back on track with her kids and being a good mom, so not too much to loose there!
Begin to document everything so that you have a good log when it comes time to go to court!
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