What valiumredhead said.
It's just a bed. And I can't believe so many of you are saying "no-one sleeps in my bed" without considering any circs where it might be the right thing to do to give up your bed for an elderly relative.
Sure, there's no need to give up your bed if there is a perfectly good (other) bed (eg SIL's) for your guest and MIL is happy with that. But I would always make sure my guests (the ones I liked at least) were well looked after. If this meant giving them my bed, then I would do that, if it was obvious they would be better off in my bed than the alternative bed. (Also depends on whether or not you made the invitation or they just announced they would come!)
I have given up my bed/room in the past without any sense of being deprived. (When I have invited people and decided to give them the best room.)
It's up to you to decide whether ur MIL is being unreasonable, (or not), cos we are not in any postion to judge.
It's basically an evaluation of your need (to stay in your own bed) versus her (possible) need to be in your bed/room rather than in SIL's.
Only you can decide but I think it is possible that MIL is not being unreasonable eg if she struggles to get to SIL's room.
It's your call, but surely you would give up your bed (and even sleep on the floor) if someone (dear to you) was in dire need and if there was no alternative?
Where is your charitable side at Xmas? (You and all the others who said "no way"!)
Shame on you all to be so overtly selfish ... and at Xmas too!