YANBU @Georgiethegorgeousgoat ... And IMO it's very little to do with the food/turkey size etc, it's just nice to know, and you are quite entitled to have an answer, seeing as how it's YOU hosting it. You can ask for an answer now for no other reason that you want to know, because it will just settle you, and you can get organising!
We already have our December/Christmas sorted. Our (adult) kids who live half hour to 40 minutes drive away, are visiting for the day on 15th December, (with their partners,) when we will put on a big buffet for them... On 21st December, we are going to the pub (me and DH and the two kids and their partners,) for a traditional Christmas meal, and we will all swap Christmas gifts.
Then 26th December, we are going to a boxing day buffet/party at a friend of the family's house, who loves to host, and is having 70 people on Boxing day. Our kids and their partners and this friend's neighbours, work colleagues and family will be going to too.
Christmas day by the way, both our kids are going to their partner's families, and it is just me and DH all day at our house. Looking forward to a day of being very lazy, and eating trifle and crackers and cheese and drinking port. No-one to cook for or do anything for ... Bliss....
Then me and DH are going to a party on new years eve at our local pub 20 minutes walk away.
We all have it in our diaries, and all settled, and did have by the third week in October. I think it's really odd to not arrange stuff like this, and give a definite yes or no, (and expect one) within a few days of being asked. Especially with it being such a busy time!
It's so damn rude to not give a straight answer. Some people have busy lives and like to be organised several months in advance, and some have naff-all responsibilities, and all the time in the world, and pretty much think the world revolves around them. (Your MIL sounds like this!) Get your DH to TELL his mother she has until the end of this weekend to give an answer - one way or the other - or you will take it that she is not coming, and will not include her in anything.
Pay no attention to people saying you're being 'precious' or impatient. These must be the same people who dilly dally and faff, and make people wait weeks and weeks for a response! Very annoying!