Is expelling wind in a gathering considered forbidden because it is a characteristic of the people of Lot, or is it merely an inappropriate act?

What your brother does, releasing wind among people, is a breach of decorum and bad manners. It is forbidden if it causes harm to those present, due to the Prophet's (peace be upon him) saying: "There should be no harm nor reciprocating harm." Furthermore, it contradicts modesty, which is a part of faith. It has not been authentically reported that this act is attributed to the people of Lot, but it is not from the etiquette of the people of religion. One who is compelled to release wind should move away from people, and if it happens unintentionally to someone, it is a matter of decorum to overlook it.