Depends who's paying!You can usually head off a lot of faffing by making it clear when you confirm the menu in writing that you've costed the menus based on particular specifications that you and the customer have agreed, and that it may cost more if you make changes.
In all the venues I've worked in, I've made it a rule to have one person who can authorise changes for a particular event. In my experience usually this is the bride, even if the parents are paying, and often I've raised it in a jokey way by saying something like, "OK, now if the groom phones me asking for beef jerky and pickled eggs to be served with the reception drinks, who should I agree it with? Would that be you, Carole (bride)? OK, so any changes to the menu will need to be agreed by you, is that right?" and then make sure that that's documented in writing somewhere.
Couples don't mind a bit of joking about the bride keeping the groom in line, and if you can agree right from the beginning that any changes to the menu have to come through the bride you've made a good start.
My first response to requests for changes was always a polite and friendly, "I'm sure that won't be a problem. Let me get a price for you and call you back". Ultimately, I believe it's fair for a customer to pay for your time if s/he causes you significant extra work.
Of course, if you've agreed beforehand that the bride is in charge of any changes, your response to the bride's mother becomes "I'm sure that won't be a problem. Let me get a price and call Carole back".
Stay away from cooking family favourites. Aunt Violet's chopped liver or Uncle Harry's killer chilli simply never tastes the same when you reproduce it in bulk in a professional kitchen, and you're on a hiding to nothing if you agree to try. Polite refusal is the only sensible response - couched in terms as diplomatic as possible!