Is wearing tight trousers by a woman considered imitating men, even if she doesn't intend it? And what is the ruling on wearing them in front of her brother or father?

A woman is forbidden from wearing tight pants, even if she does not intend to imitate men, because they define the size of her 'awrah and the details of her body. She should only wear them in front of her husband. The actions of people, no matter how widespread, cannot be used as an argument, nor do they justify what the Sharia deems abhorrent.