Is it permissible to wear trousers in front of one's mother's sister's husband, who is like a father and stays in the house for weeks? And is it obligatory to wear the hijab in front of one's mother's sister's son who was born in 1990?

If the husband of the maternal aunt reaches a stage where he no longer desires women, then it is permissible for him to see what a mahram (unmarriageable kin) is permitted to see of his mahram. However, if that is not the case, then it is not permissible, due to the convergence of texts prohibiting a non-mahram man from looking at a non-mahram woman, as Allah Almighty says: (Or those male attendants having no sexual desire). It is also not permissible for the questioner to wear tight trousers in front of men, with the exception of her husband. As for a child, if he shows inclinations towards women, he is like an adult man in ruling.