Is it permissible to give vowed charity to needy relatives (such as siblings and the wife's family), or must it be given to non-relatives?

What you uttered is a conditional vow (nazr mu'allaq) that you are not obligated to fulfill unless pregnancy occurs. If pregnancy is confirmed, then you are obliged to give the mentioned amount in charity. You are permitted to disburse it to your siblings and your wife's relatives if they are poor.