I said that I'm not going to talk, but I kinda have an idea for Fate and Cyanide's relationship as brothers.
Of course, Fate manipulates his easy target, his brother Cyanide, both emotionally and mentally, or physically.
For example. Fate can damage Cyan's thinking of him as a bad brother and can make Cyan turn on his allies. That way, he won't let him interrupt his "fun" with his alone time. However, Vanessa strongly dislikes Fate for what he's done as Fate forced him into an inseparable pair.
Cyanide is the most vulnerable angel who can't break the rules, even following Fate's trap.