I've name changed for this one!
My step SIL (to be) on DFIL' s side (MIL and FIL are divorced) is arranging my hen do.
On Monday, MIL told SIL that she was bringing her 11yo with her to the hen do and they'd be leaving at 8. Not once did she ask if she could come.
I don't want a child at my hen do. It's an adult event and I want to let my hair down. I messaged MIL very nicely to say 11yo isn't invited as it's a hen do. MIL replied no problem.
SIL has informed me that MIL has messaged her saying she won't be coming anymore and she doesn't understand why her own mother can't be involved (DP's Grandmother). So basically MIL is no longer coming because she can't bring her mother and 11yo daughter along?
What's really pissed me off is that the date was rearranged to suit MIL as she kicked up a fuss about the date. This has resulted in my best friend not being able to come as she's on holiday on that date.
So now MIL isn't coming and my best friend isn't either 
AIBU? My DM has suggesting to allow the 11yo to come along to keep the peace but why should I? This is my hen do not MIL's.