Should I donate the money I intended to donate to Palestinians to my needy relatives, given that I cannot donate to both parties?

Fulfilling vows is a characteristic of the righteous, and Allah has praised them for it. A vow made with money for charitable causes must be fulfilled, especially if it is for a specific recipient, such as our brethren in Palestine, for they are deserving of zakat, charity, and vows. If you have vowed to a specific entity, you are obligated to fulfill it for them. As for maintaining kinship ties, strive to do so with whatever is available, other than the vowed money. If you are unable to do so, there is no blame upon you.