is your mum one of those people who might ask you a question in the hope of starting an argument? cos asking a vegetarian if they want chicken or beef... yeah... how well does she expect that discussion to end?
i'd feel inclined to accidently on purpose miss-hear her, and say "chicken or beans?- i'll take the beans please!
this is your brothers wedding, right? the bride is gonna be your sister in law after this? i dunno... surely they could rustle up a salad for the grooms sister without it being a huge deal?
lay it on thick.
if all else fails, you could threaten to get out your cunningly concealed lunchbox right in the middle of dinner, and start snacking on hummus and cucumber sandwiches, tofu and hemp smoothies, and al sorts of other weird (and no doubt embarressing for the hosts) veg*n foods, right infront of everybody.
would it help to totally detour your mum, and talk to your bro? do a sisters speciality sucking up trick, or offer to phone up and sort it out with the caterers yourself, and cover any extra costs, so its no trouble to them?