What is the ruling regarding a fiancée marrying her first fiancé, or the one who committed zinā with her and repented? And would she bear a burden of sin if she were to marry the first one?

The sister has done well by repenting. It is not permissible for her to marry the man with whom she committed adultery unless he repents. If he repents and she remains undecided between marrying him or another man, she should choose the one who is better in religion and character. She must be careful not to be lenient in dealing with foreign men, such as unlawful seclusion (khalwa) and prohibited mingling (ikhtilat).