What is the ruling on fulfilling the promise to marry one's paternal uncle's daughter after engaging in prohibited acts with her, then becoming repulsed by her and having the family reject her proposal?

Fulfilling a marriage promise is not obligatory, and it is permissible to break an engagement for a valid reason. The non-obligation of marriage is further confirmed if refraining from it entails obedience to parents, unless one fears falling into sin. Marriage is advised for the preservation of one's religion and chastity.