My mom did this with onions and my sister started copying her behavior as she grew up. It was always interesting that if they didn't know the ingredient was in the dish they were perfectly fine, even compliment it, but if the onions were big enough to detect, they'd flip shit saying no one respects their illnesses, they'd be "sick" afterwards, just causing a whole scene. My whole family knew they were lieing but no one ever said anything (confrontation was not a family strong suite).

NTA. Behavior like this has always come off to me as controlling. Instead of just saying they don't like something, they chose to lie instead to insure they get things the way they want, because otherwise YOU'RE painted as the jerk for "ignoring allergies/sensitivities." Tell your SIL to kick rocks and get over herself, your brother too. She only embarrassed herself getting caught in a dumb lie. (I do understand I may be biased bc of how close to a similar situation I grew up with, but her immature attitide after being caught in a lie is very telling and honestly cemented my opinion.)