Definitely don't give up yet! I'm sure lots of people are going to give you great advice and I can only tell you my experiences and what has worked for me. I have several cats of my own but I also foster kittens. The first time I brought home a pair of kittens to foster (5 wk old ferals lol), it wasn't really practical for me to set them up in a seperate room so I blocked off a corner of my living room where they could be contained. My cats definitely weren't thrilled. My female especially would hiss at them when she saw them for the first few days. I just continued to act totally normal--if you stress your cat will sense it and feel even more like something is wrong!
After they had tamed down (only took a few days since they were so young luckily) and I felt more comfortable I let the kittens out of their "corner" into the rest of the living room, supervised of course. My cats had other rooms and lots of high perches to retreat to if they felt like the kittens were getting too close. For a few days, whenever the kittens got too close to the cats, the cat would reach out and "pin" them to the ground with a paw (no claws)--you could say this is an adult cat's way of teaching a kitten manners lol.
I don't remember exactly how long it took, but I know it wasn't more than 2 weeks before the kittens were able to snuggle up to my adult cats who would then groom and cuddle them. I now bring home foster kittens on a regular basis and my cats are really so great about it--I

them for it. They don't necessarily "take" to every kitten immediately, theres still an occassional hiss and pinning to the ground, especially with particularly rambunctious kittens.
I have also had success using a large metal dog crate for new kittens and placing it in the living room. I drape a blanket over it leaving only one side exposed and make sure theres a covered sleeping spot for the kittens so they don't feel too overwhelmed by curious cats surrounding them. I leave them in the crate for several days or even longer if they are very young. This way the kittens get used to all the new smells/sights/sounds/etc. of their new environment but are protected. The cats become accustomed to the presence of the kittens in the room without feeling as threatened by them.
These are just some things that have worked for me and what my experiences have been. Cats are all so individual that I don't believe there is really one specific way that is going to be fool-proof for all cats. Since Kiki seems to be feeling pretty freaked out right now, my advice would be to take a deep breath and try to stay calm yourself for her sake. You already have a kennel set up for Whisper? I would keep him in there (with everything he needs of course), maybe in a room where you spend alot of time, maybe with a sheet draped over at least the top, back, and sides. Go about your normal routine with Kiki. Since she's acting defensive don't force attention on her but you might be able to entice her to come to you with treats or interact with you with a favorite toy which might help her relax.
Although there *are* those cats that absolutely will not accept another cat I think they are definitely the minority. Some cats might stay more aloof from other cats but I definitely think that with time and patience most can learn to at least live peacefully in the same house, and many will eventually become close friends and playmates.
How young is Whisper? I've never used Cat's Pride litter--is it clumping? I've definitely had problems with clumping litter getting stuck in small kittens back paws. Actually clumping litter isn't really recommended for young kittens, theoretically it could cause intestinal blockage if a kitten ingested too much by grooming itself or simply eating it out of curiousity. So maybe try switching to a non-clumping litter for Whisper until he's a bit older? Maybe WBCL? If it still sticks to his feet I'd just watch him closely and help him remove it. Baby kittens just aren't that good at staying clean and grooming themselves yet.
Good luck! I really believe you can do this!