What is the ruling on vowing to offer a "kid" to Allah if the wife gives birth to a male child, and is it permissible for any relatives to take a portion of it, or must all of it go to the poor and needy?

Your relative is not permitted to speak of the Unseen, which none knows save Allah, and you are not permitted to believe it. As for the vow (nadhr), fulfilling it is obligatory if it was made, unless it involves an act of disobedience. If the vow is absolute (not specified for a particular recipient), then it is like charity for the poor and the needy, and your relatives may eat from it if they are poor. It is also said that the one who made the vow and his family may eat from it unless it was specifically designated for the poor. However, if what emanated from your brother was merely a promise of a vow, then nothing is binding upon him as long as he has not actually made the vow.