I was hospitalised during both of my pregnancies for severe sickness andstill remember (10 years later) just how awful I felt. Dp was wonderful, as was MIL, who, once I was out of hospital would bring round pots of whatever it was I was fancying and not complain at all if I couldn't eat it, threw it up etc.
I suspect MIL feels a bit as if she's trying to "jolly you out of it" whereas we all know that you're not choosing to feel like that and would probably love to be able to be out and about.
And I think that's the key really - say you love the sound of it and that you'll let her know closer to the time - and then back out of it. Or even, if you could on the day, go along for a short time, smile sweetly and then clear off home again.
Getting out for a while might help, at least temporarily, though I remember dragging myself along to a family party, feeling awful, leaving early, then being sick all the way home, I was still glad I'd gone, as it made me feel almost human for about an hour, rather than just a creature at the mercy of her body, as it were.