I'll try to stick with facts, and avoid my side commentary on this...
IL's rented a beach house for a week, with one extra bedroom.
6 hour drive from us.
Invited us (me, dh, preschooler, baby) to join them for 2 nights.
We agree.
3 weeks before the trip they send us the details on the house (we had been asking for months).
The extra bedroom has 2 twin beds, with about 2 feet of floor space between them (remember we are a family of 4).
The other bedrooms in the house are no bigger.
WWDCUM Do?
Renting our own house is not an option - we are in a financial tight spot and were planning on foregoing a summer vacation this year.