If she is stealing from you to support her habit then the only thing you can do is to get the authorities to intervene on your behalf. My ex sister-in-law has done this to her family for the past 15 years and the only thing that helped was a court ordered drug treatment stay.
Are the children being taken care of? The children will suffer in the household of an addict even if they don't seem to be from outside appearances. If they let you I would spend some quality time with the children, they need it.
I agree with dovt on the stealing issue, although if you get the authorities involved it might close the chance of helping the children.....
It is a painful situation for sure for all of the family I would attend alanon or narconon to get support and pamphlets that would help.
PS anyone know how to spell narconon (I'm an addict, alcoholic, I never went there)
Tough love isn't always easy to practice, but if you don't start to do it right now you will simply be enabling your step sister to continue using drugs.
You will be doing her and her children a huge favor. The kids need someone to advocate for them-let the adults take care of themselves. They can either choose to get clean or suffer the consequences of choosing to continue with the drug use.
Calling the police if they steal again is what you should do. If the children are not being properly cared for it behooves you to call CPS/DSHS or whatever agency it is in your state that looks out for children.
Good luck-keep a positive outlook on things. By practicing tough love and making the hard choices, you may very well save your step sisters life.
Greatgreebo
The sentence "you will be doing her and her children a huge favor" should have "by starting the tough love now" added to the end :-)