long story short please help if you have any experience with drug addict family members?

my sister inlaw is a meth addict and who’s kids were taken away because of neglect and were put in foster care where they have been for 4 months now she was told if she cleans up her act and gets into a program that she could get her kids back in nov. her son just died he was 1 in foster care and she burried him and the girl 4yrs is still in there. the death under investigation because it looks like he was abused by the foster parent! she has been clean these 3 weeks since her sons burial and yesterday she went back on it and i am so frustrated because she said she wouldn do it and when i turned my bach she did! what can i do to help her get her shit together for her little girls sake and her own she is only 23 so she can still turn her life around and be a great mother! what should i do?