Is a son sinful if he is unable to cover his sister's marriage expenses, as requested by his mother, while his wife refuses to contribute, and his financially comfortable father is present?

The questioner and his brother are not obligated to pay for their sister's wedding expenses; rather, it is an act of benevolence and upholding kinship ties.

The wife's salary is her right, and she is not obligated to give any part of it to her husband unless it was stipulated beforehand or she offers it willingly.

There is no sin upon the questioner for refusing to pay what his mother asks for, but it is advisable to strive to please her as much as possible, for honoring her is among the greatest acts of drawing closer to Allah.

A man's marriage to a second wife is not, by itself, a justification for discord and separation, and it is not permissible for the wife to seek to sever ties between the father and his children.

The sister's marriage is only valid through her father. The preponderant opinion is that the marriage is invalid if someone else marries her off while her father is alive, unless the father has lost the eligibility for guardianship or is unjustly preventing his daughter from marrying.