E.J.
This is horrible communication.
How I read this is that Mary is coming into town with her son and found a place halfway between you and Jane.
This is not a 'Girls' weekend, JMO.
A ladies get away, to me, would not include kids and would include a date and destination picked by the ladies going....no kids...no significant others...and no...coming into "visit" anyone.
It sounds like Mary found a place to stay in town and invited you all. It's up to her to set the parameters of this visit, which in fact she did by including her son.
Since nobody defined how this weekend was supposed to be, and since only Mary booked the place she is staying at, you need to let the husband thing go.
You all need to define what you mean by 'get away' and take it from there.
But in my opinion 'a place to stay halfway between households' does not equal "get away".
ETA: I agree with Marda...what does this have to do with 6 friends from college.
LOL, feel like I just read a mathematical word problem: 6-3+1husband +5 kids divided by 4 adults divided by half the distance of the two households......