Great pictures of all of your orange cats in your "orange tree".
I'm sorry to hear that Radar is so sick.
Maybe Pearl sensed something was wrong with him and that is why she hissed at him.
She and Ziggy may also be feeling overwhelmed by all the new orange cats in your home, sometimes it takes a while for cats to get used to just one new kitten.
Please give some hugs and kisses to all of your cats for me.