If Hisoka tries to harm Killua or Zoldyck family, yes we will. Hisoka probably wants to fight him eventually, but Illumi doesn't seen to.
Illumi is rlly twisted but he Kills out of profession and general purpose for his goal. We know Hisoka lives to fight strong opponents around the world and crush them, Illumi is like his only friend besides Gon. But Hisoka rated Illumi a 95 somebody with an aura that intrigues him very much, somebody very close to his level of skill. All Hisoka rlly cares about is the future and the amount of toys he can have to play with (Fight with) and only breaks them when he chooses they are boring and not gonna be able to entertain him anymore.