i think your mother in law is being too much by complaining to your sister In law! if she complain to her own family members or relatives, i got nth to say, but lik what you said, your sister In law is your bro's wife. By complaining to them how 'bad' the sitting arrangement was, also indirectly blaming them for such 'lousy' arrangement..
i dont see anything wrong for you to sit on the VIP table since the one getting married is your BROTHER! your parents has been very polite to invite your inlaws yet your mother in law created such a sence.
you r lucky that your hubby stands by you. at least he knows what's going on and most imptly, he know his mum's 'pattern'..