Tell the truth! Who doesn't want to let the new DIL host the holiday?

I know it's easy to say, but try not to make too much of this. Women get weird around the holidays - especially widows or divorcees. You didn't mention a FIL. Is there one? There are more family arguments around holiday time than any other time of year. Try to ignore it and enjoy the day. Who knows what she was thinking? It could be a million different things. Maybe the tenant and his daughter didn't actually accept her invitation and later they finally refused her. Maybe she's depressed and feeling like she wanted to celebrate at home.
As for your initial question, I love when my DIL invites me for a holiday - or anyone for that matter. I usually get the honor of cooking for 20-30 people each Thanksgiving, Christmas and Easter! I love anyone who invites me at any time of year. This Thanksgiving my daughter is taking over at her apt. and I can't wait to go early in the morning and help her.
Have a great Thanksgiving.

It sounds like your MIL is very old school and very unhappy. I wish I had some advice for you, but it's not easy when someone is widows and feels shut out of things. I would let your husband make decisions as far as she is concerned and try to stay by his side. I wouldn't want to be in the middle of this.
Good luck.