i think your should better prepare a few plastic bags in the car. last time when i preggy, even till last tri when i seldom vomit alrdy, i still will put 1-2 plastic bags in my bag. during the 'peak season' of my MS, the 1st 5mth+, i had AT LEAST 4 plastic bags when i go out. cos sometimes i will used up 2-3 plastic bags when i travel to one place.. haha! that's y that time cannot take bus/train, must take cab. haha!
as for your mother in law, if me horr, i sure pull my hubby, hint hint him to tell mother in law. or just keep go toilet puke. HAHAHAHA!