When is a brother obligated to financially support his brother or sister or others who are not among his ascendants or descendants?

The jurists have agreed on the obligation of maintenance for ascendants and descendants when there is a need, while they differed on the maintenance for other relatives:

Hanafis: They made it obligatory to provide maintenance for every relative within the prohibited degrees (Dhawu al-Arham al-Maharim), such as a brother or an uncle, arguing that withholding maintenance leads to the severance of ties with these prohibited relatives. Hanbalis: They made maintenance obligatory for every relative who inherits either by a prescribed share (fard) or by residuary right (ta'sib), subject to three conditions: the recipient’s poverty, the provider’s ability, and the provider being an heir to the recipient. Shafi'is: They restricted maintenance to ascendants (fathers and grandfathers) and descendants (sons and grandsons) only. Malikis: They did not make maintenance obligatory for the paternal or maternal grandfather, nor for the son's child or the daughter's child.

The most preponderant opinion is that of Imam Ahmad.