Bisky has sent Killua and Gon on a mission to become allies with none other than Hisoka's younger siblings yet Bisky never told them that part

Hisoka's sister was in terrible condition for a while so Killua and Gon were sent to the hospital she had been at only knowing to become allies with this person once they saw her and now they were walking in a large hallway with immense nen like Hisoka's was but much more sinister

Killua: "I think that's her..."

Gon: "That's a familiar nen..."