Well it could go lots of ways couldn't it?
Scenario 1- Nothing is said, both families there, respectful boundaries/privacy etc. All ok.
Scenario 2- OP says something, no offence taken, agree to boundaries/etiquette/privacy
Scenario 3- OP says something, massive offence taken, no dates changed, awkward, ruined holiday.
Scenario 4- OP says nowt, horrendous holiday, other family in their face every minute, probable lost friendship on return.
Scenario 5- OP changes dates, keeps it quiet, comes up with vague plausible excuse nearer the time, fab holiday, friendship intact.
I'm with number 5 all the way
(feel free to add more scenarios)