Is zakat obligatory for relatives if they are well-off, such as a daughter and her husband who live independently in their own home?

You are not obligated to pay Zakat al-Fitr on behalf of your married daughter. Rather, it is incumbent upon her husband, if he possesses the sustenance for the day and night of Eid, in excess of his and his family's needs. He should pay what he can afford. As for Zakat on wealth, it is only obligatory upon one who possesses the nisab (minimum threshold) and a full year has passed over it. It is permissible for you to give your Zakat al-Fitr and wealth to your daughter's husband if he is poor.