Is the wealthy brother obligated to financially support his poor, widowed sister who lives with him, or is what he provides considered a debt upon her?

Scholars have differed regarding the ruling on the maintenance of relatives. The preponderant view is that it is obligatory for every relative who is an heir, and this is the madhhab of the Hanbalis. Since the matter is one of dispute, the issue should be referred to the Sharia judge to rule according to what he deems preponderant, in order to resolve the dispute.