MIL nonstop chortle

I bet it's an anxious/nervous habit. When I was a teen and in my 20's I had a terrible time with nervous, inappropriate laughter. I would do everything I could do to try to contain it but frequently it was still pretty obvious. People would sometimes accuse me of being high-I wasn't-just anxious. Thankfully, now that I'm older, that's resolved but I still have trouble with other nervous habits (silences are tough for me...I get nervous and do whatever I can to make conversation...I'm working on it).

I'm sure it's annoying, OP, but keep reminding yourself that she doesn't mean any harm. She may not even be aware that she's doing it.