I'm so sorry you are going through this. I grew up with an alcoholic parent and worked with IV drug users for a few years. Rough population to work with, specifically due to the serious personality changes that accompany addiction. there were people who actually admitted to routinely stealing from their 5 year old's piggy bank to buy drugs and then complained and cried about how their kid didn't trust them. no joke.
addictions often start out through self medicating for other problems in their life - she could actually be clean for her drug of choice but stealing for another type of addiction (it is common for addicts to jump from addiction to addiction). Shopping, gambling, alcohol, prescriptions that might not show up on routine tests, etc. Some thieves also just really love to steal, they are looking for a high in life and stealing helps them achieve that. There is also a psychological dysfunction that can develop for long term users and individuals living in other dysfunctional situations - even if they are clean, its almost like they lose the ability to abstain from instant gratification and often continue to do really really dumb or dangerous things even though they are not technically under the influence. Kindof a use it or lose it situation - if you aren't used to doing the responsible thing, it is much harder to do it. combine all of this with mental illness like depression or bipolar disorder and you have a complete disaster.
understand that by letting her stay, she will most likely continue to abuse the family somehow. someone who is this broken, regardless of the reason, will continue to be a harm to self and others. only you know where the true line in the sand is for you. is it another theft? drugs in the house? neglect of the child? think about the line. hold it when the time comes.
if you ever need to truly kick her out, provide her with resources to local women's shelters, cps, etc. again, many people in these situations can't actually think clearly and even just having a piece of paper with some numbers to call can be a life saver once they get a moment of clarity.