To not want MIL to come on my birthday outing?

[Rosemallow]

Ok so it's my birthday this weekend and we've decided to have a nice 'family' day and have bought tickets to an event. (me, DP, DD and DS)
Today DP informs me that MIL is now coming and because she's is going to bring a picnic that is ok.
TBH his mum is a little bit weird and not particularly fun/helpful to have around. In fact she is very hard work.

She NEVER interacts with the DC and has been known to just let DD wander off when supposedly watching her (once she nearly fell down a flight of stairs and I had to run over while 8.5 months pregnant to catch her!).
She will sit around talking about herself and the company she works for the whole time she is here and will not listen to anything anyone else says.
DP has really pissed me off this time - she always comes on days we have arranged to do things as a 4 (DD's birthday, Christmas, my birthday last year) and it also means none of the other grandparents (including FIL) get a look in. It's not even as though she is on her own, like my mum as she has a partner of 25 years.
AIBU not to want her to come?

Make sure you spend the day interacting with YOUR friends. Is there some way you can engineer some split of the party at some point so you leave her with him for a couple of hours? Then smile sweetly later and say, well, I thought you'd like some special alone time with YOUR guest, I mean, if you'd wanted it to be a special day for me and spend time with me on my birthday you wouldn't have been the one to turn it into a party, eh?

And make sure that when the day is referred to afterwards, that you say 'What, my birthday? Well I didn't really HAVE a birthday day in the end, did I? You invited your mother and then we had to just turn it into a random outing, don't you remember? My birthday day got cancelled as far as I can see...'

And next time he does it (because he will, because you have effectively let him off uninviting her) make things AWFUL for him. Flat choice - uninvite her within earshot of you (to make it clear that he's not blaming you for the uninviting) or you and the children will have something to do at the last minute and he gets to spend the day alone with her.

(I understand why you've let him off this time though - it's your birthday so you hardly want to end up doing nothing yourself!)

I'm never sure why some people find it hard to just tell people they have other plans. Different if you haven't and just don't fancy someone coming round but in this case you had plans to a ticketed event and saying "oh well as I said we're out then, it'll have to be some other time" when someone moans they planned to visit you at a busy time would be easy to me.
We're rarely in at weekends so visitors have to be planned in advance, and agreed by both.

Sometimes when I read these sort of threads, I realise how lucky my DD is .

I wouldn't dream of inviting myself along to her family outings. We had our family times and now it is time for her and DH to develop their family memories and traditions.

Her MIL is the same way.

We wait until we receive an invitation.