Is boycotting a wedding – including the severing of kinship ties that entails – permissible in Islamic law, for the purpose of mending family relations, especially with parents?

Your uncle has committed a grave sin by severing ties with his parents. This is one of the greatest major sins and an act of disobedience to parents (ʿuqūq al-wālidayn), and he is considered to have committed an egregious act of wickedness (fisq shanīʿ). You must advise him and guide him to be dutiful to his parents, and you should not cut him off.