Whether they are married or not is still a mystery. And they are married so that when/if hisoka dies illumi will inherit the money. But apparently hisoka decided to edge himself so it’s only a matter of time before he reaches his breaking point (even his night spent in front of a tree to, uh, take the edge off isn’t.
Who is hisoka married to? So the the manga stream version sorta. Illumi unironically described their relationship as a marriage.
In the last tome of hxh, illumi is talking about a reward he could get if he kills hisoka as an “engagement ring”. The third clue concerns a fan theory that illumi changes his style after meeting hisoka. This is one of the greatest mysteries of this century.