The only questionable thing LW's brother-in-law did was to say "I'm bringing my boyfriend and he will need this specific food" rather than asking to bring his boyfriend, and then mentioning what the boyfriend can or can't eat..

On the other hand, if this is an established partner, not "I'm bringing my new boyfriend, he just got out of the hospital," the LW and her husband should have invited the partner along with the other in-laws.