What is the ruling on vowing not to propose to a girl before a close female relative, and is it permissible to retract it if it was made at the age of 17, and is contemplating this considered a lack of modesty before Allah?

You are not obliged to marry your relative, and you may propose to someone else. You should choose a religious woman, for if you find someone more religious, then proposing to her is more befitting. It is more prudent, if you break the aforementioned pledge, to offer expiation for an oath, because if one commits to a pledge concerning doing or refraining from a permissible act, it is treated like an oath.