Should mother in laws prepare meals when visiting on vacation?
There is no etiquette rule on this. It depends on the situation.
When visiting someone as a house guest, it is always nice to offer
to provide some meals, either by taking the hosts out or by
preparing them yourself.
However, different people have different feelings about using
each other's kitchens. If the couple hesitates at the offer, the
visiting relative should quickly change the subject, as it is a
clear indication that they are not comfortable with someone else
using their kitchen. And of course, the host couple should never
suggest that the house guest should do any sort of house work while
visiting.