I don't think you're being mean in not asking her. Only you know whether she would 'fit in' with both the activities and other people going, so you are best placed to make that decision.
It seems to be just as common to invite MOB/MOG as to not invite them from what I see on here. My friend had her MOB/MOG at her hen do and it was fine (though they left the evening bit earliest!) and I didn't have MOB/MOG at my own, which again was fine.
I think it's a bit rude of anyone to presume they are coming (and then to throw a strop when they discover they are not!!) and in this sense your MIL has only set herself up for disappointment. It's your hen night and it's up to you who goes. I agree you need to try and be tactful about it though. Perhaps tell her that you were limited on numbers or trying to keep numbers down to make logistics easier?
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Mrs BV since 14 July 2012