Wow, I can see your problem. They are supposed to be anonymous and if your MIL is there, you can't be anonymous. I wouldn't worry about the effect on your AH, I'd worry about yourself and what stories she might tell about what you'd say. It would definitely be inhibiting.
If there are no other meetings, and you can't expand your area to include other towns, can you try al-anon meetings? Much of it is the same, and most al-non meetings welcome people who's family members use drugs. And if that doesn't work, can you go and just sit and listen and not share--would you get anything out of that?