Which is better: sponsoring an unfamiliar orphan, or sponsoring orphaned siblings who are already sponsored by a charitable organization? And would one receive the same reward for sponsoring an orphan in both cases, given the questioner's limited resources and need for money?

Sponsoring an orphan is one of the greatest acts of worship and obedience, through which one attains immense reward and companionship with the Prophet (peace be upon him) in Paradise. If your siblings are already sponsored, then seek out another orphan who is not sponsored to sponsor them.