Is there anything that would absolve a person who swore an oath on the Qur’an to love a girl as his sister until death, but then his circumstances changed and he became able to marry her?

Breaking an oath is not unlawful unless the oath was to perform an obligation or to abstain from a forbidden act. The details are as follows: If one swore an oath to do something disliked (makruh) or to abstain from something recommended (mandub), then breaking the oath is sunnah (recommended) and fulfilling it is disliked. If one swore an oath to do something recommended (mandub) or to abstain from something disliked (makruh), then breaking the oath is disliked and fulfilling it is sunnah (recommended). If one swore an oath to perform an obligation or to abstain from a forbidden act, then breaking the oath is forbidden and fulfilling it is obligatory. If one swore an oath to perform a forbidden act or to abstain from an obligation, then breaking the oath is obligatory and fulfilling it is forbidden. As for swearing an oath concerning a permissible matter, one has the choice between breaking it and fulfilling it, though fulfilling the oath is more preferable. Based on this, if you swore by Allah not to marry that girl, then you are not obliged to fulfill the oath, nor is it forbidden for you to break it. However, you will be required to pay an expiation for the oath if you break it and marry her.