How should I act between my parents and my fiancée, whom they reject due to problems with her family, given my fear of disobeying them and of wronging her by leaving her after a two-year engagement?

Obeying one's parents is obligatory, and disobeying them is a grave sin. If the parents reject a girl for acceptable reasons, then it is obligatory to obey them, unless doing so would cause significant harm to the son or lead him to commit something forbidden (haram (definition)). However, if their rejection is for no acceptable reason, then it is recommended to obey them, but not obligatory. In this case, it is permissible to marry the girl while being keen on honoring them. It is important to note that the suitor is considered a stranger to his fiancée until the marriage contract (aqd) is performed.