Hi Jan
Welcome... wow, what a mess.
Couple questions might help.
First - how much does your husband talk with his mother [your MIL]? Does your MIL know what has been happening, at least from his perspective?
Second - how 'together' is your MIL? Do you think she can handle being told the facts, or is she likely to reflexively defend your mother merely because your mother's your mother? ["Bad Mommy Taboo" in action]
Third - how much time do your mother and your MIL spend together otherwise? Does your mother have any leverage over your MIL [is she the president of the garden club and your MIL the VP, for instance]?
If your MIL is a reasonably sensible lady, and your H has given her some idea of what's been going on, and you don't think she'll react with knee-jerk defensiveness of your mother, you might want to take her out - lunch, dinner, coffee, whatever - with your H, this is important - and fill her in. Both you and your H together doing this, if possible, so she sees her son supporting you. And also gets the message loud and clear that you both know she's not the same, that she's not at risk of losing you.
You can then encourage her not to be conned into playing the peacemaker role, but to refuse to participate in any triangulations your mother attempts - she can just say, "Well, Lydia, [or whatever your mother's name is], I really don't know anything about this, and it's not my place to interfere. You'll need to sort that out with them."
Your mother will probably push for a little while, trying to get her way, but if your MIL maintains her boundary, this will eventually stop.
Unfortunately, the only really effective disinfectant is sunlight. The easiest way for your MIL to understand that it won't help for her to get involved is if she understands why...
Wishing you luck and lots of strength.
