My younger sister is a meth addict, too. She has a daughter, almost 4 who my parents are raising. My sister is out of the house with an addict bf. They flop from house to house. Until they are ready to get help, there's nothing we can do except keep her daughter healthy and safe. My parents are looking into temporary custody of their grandchild.
What has been working for me is Nar-Anon meetings and being the best auntie I can be for my niece. My parents are financially strained and emotionally drained, too.
As I keep hearing from recovering addicts, they best thing we can do is stop helping/rescuing the addicts in our lives. They will only continue to take advantage as long as we allow them to do so.
Praying for the children involved!