Did I commit a sin by changing the division of the first vow? And is it permissible for me to change the beneficiary of the first vow from the neighbor to the youngest uncle due to his straitened circumstances?

A vow must be fulfilled in the manner it was made, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." If you vowed to distribute the tips you receive in the manner mentioned in the question, then you are obliged to fulfill it. It does not matter that it is irregular or unstable, nor that your neighbor is not needy.