All perfect praise be to Allah, The Lord of The Worlds, and may Hus peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The vow made to do an act of obedience for Allah must be fulfilled by a Muslim, since the Prophet (PBUH) said (What means): ”If anyone vows to obey Allah, let him obey Him.” [Related by Al-Bukhary No./6696]. Further, giving charity to your orphaned nephews is an act of obedience that must be fulfilled i.e. if you (Questioner) made a vow to give them charity so long as they need money or till they grow up, then you are obliged to fulfill your vow literally and abide by the period. And Allah Knows Best.