What is the sin incurred by the maternal aunt and whoever participated with her in wrongdoing and cutting ties of kinship, and is there a sin upon the questioner if he ceases attempts at reconciliation after her repeated rejection?

The aunt is sinful for her estrangement and abuse, because estrangement is forbidden and abuse is a clear sin. If you greet her and she does not respond, you are absolved of the sin of abandonment, and she bears the sin. We advise patience, kindness, supplication, seeking refuge (with Allah), remembrance (of Allah), and maintaining prayers.