RosellaStar
Diluc could lie to everyone, to the whole wide world, but no matter how much he wished he could, he could never lie to his heart. Because it wasn't just a stupid bracelet, now was it? It did't mean anything- it meant everything. It was the sole item that kept him close to his brother. It was the thing that connected them. It was their friendship bracelet and it showed the world how strong their bond was. He may have disowned Kaeya but deep down he was always his brother, his inability to remove the bracelet proved that.
TLDR: Kaeya made Diluc a friendship bracelet when they were kids and noticed Diluc still wearing it one night, after everything they've been through, leading to a reconciliation. A friendship bracelet saves the day.