Will the brothers and sisters be sinful for forcing their engaged sister to leave her fiancé, whom she loves, after they both repented of the transgressions they committed? And what should she do regarding their pressure on her to annul the engagement?

Brothers do not have the right to demand that a wife separate from her husband. If they do not have a legitimate religious justification, they are sinful, because striving to separate spouses is a grave sin and an act of sorcerers and devils. The wife is not obligated to comply with their request, especially if the husband is religious and treats her well. She should maintain ties with them honorably, and if they sever those ties, then the sin of severing kinship falls upon them.