Is it permissible for a man to give his poor, widowed sister from zakat, knowing that she has an adult son who works and may give her an insufficient amount, and that her brothers and the questioner's husband give her a monthly sum?

If a sister is poor and her son is unable to support her, it is permissible for her brothers to give her from their Zakat funds to suffice her needs, while continuing to provide her with monthly donations and maintaining kinship ties, as this is among the greatest acts of worship.