I'm pretty sure IANBU, but she's being so insistent that I'm beginning to second guess myself.
Background: we live in NZ. MIL lives about 2 hours drive from us. My family live in Ireland. We see MIL about once a month/6 weeks. We haven't seen my parents in nearly 2 years.
My parents are coming to visit for a month in about 3 weeks. We will be travelling a bit while they are here, trying to fit in a few events and see various members of DH's family (MIL included).
So, today. We mention that my Dad and DH will be taking DD1 to a show that has a special significance for my Dad and DD1. Mum, DD2 and I won't be going, DH might. MIL announced that she would be coming up to go with Dad and DD1. DH was a bit non-committal and mentioned that the house will be very full. (My parents will be sleeping in DD1's room, DD1 will be on a pull out bed in DD2's room, there is no other potential sleeping space).
MIL's response. Oh, that's ok. I'll come up with BIL, and we'll sleep in the garage on air beds. 
Our garage is not a fancy garage that actually doubles as an extra room. It's where I keep my car, and rubbish bags before we put them out for collection. It has bikes, camping gear and gardening stuff. It is most definitely a sleeping space.
But she is absolutely insistent that she is coming up with BIL, sleeping in the garage and going to this show with my Dad and DD1.
AIBU to put my foot down about it, and how on earth do I do it when faced with such insistence???