Hi Klekota,
You say that your boundary is for her to get clean and work on her issues... this is not a boundary but a rule for your AW. I can tell you from experience that rules do not work for the addict. When someone is in active addiction they really don't care about rules or anything else other than getting their drug of choice.
A boundary is for yourself, it's where you draw the line in the sand to what you will tolerate. It can only be trampled on if you allow it. For example, your wife gets arrested for possession, the boundary you have set for yourself is that you will not come to her rescue and bail her out of jail. This boundary has consequences for the addict but in turn, it helps you to restore your peace of mind.
It kind of sounds like her mother is in denial and is enabling her daughter's addiction by making excuses for her. Unfortunately, there is nothing that you can do to stop your MIL from enabling and your wife from using. You can, however, enforce strict boundaries for YOURSELF and stick to them. When I was married to my ex one of my boundaries is that I would not be around him when he was clearly high or using. I would go for a drive, stay at friends or just go to the gym and work out for a while until he either passed out or left for work.
I would highly recommend attending Alanon or Naranon so that you will gain some face to face support from people who are and have experienced the same thing you are dealing with right now. This forum is also a great source of strength.