How can one deal with people who are boastful about qualities the questioner does not possess, such as a brother who uses physical strength or a beautiful and arrogant sister-in-law, and is withdrawal and avoidance of confrontation the best course of action in these cases?

Bad manners and arrogance towards people because of beauty are blameworthy. Humility and abandoning pride are obligatory, based on the words of the Prophet, peace and blessings be upon him: "And indeed, Allah has revealed to me that you should be humble, so that no one boasts over another, nor does anyone transgress against another." It is advised to be patient and content with what Allah has apportioned, and that the true competition is in the ranks of the Hereafter, while maintaining self-confidence and dealing with the ignorant by turning away from them.