Adult cats generally don't like kittens. Kittens are chaos. Kittens are beserk. Kittens think it's fun to use adult cats as attack toys. Kittens bounce off the walls and the furniture and the other cats. Kittens steal food and crash through the food bowls and water bowls tossing things everywhere. Kittens steal favorite toys and sun spots on the rug, while they are in use. Adult cats generally don't like or enjoy the company of a kitten until the kitten grows up and slows down a bit, sometime after a year or two.
Bringing in an adult or nearly-adult cat may work better, IF all three cats are the kind who do like other cats, and they are properly introduced to each other. At least a grown cat isn't going to use your existing cats' heads as trampolines like a kitten would.
We have never had a bonded pair of siblings, but we have had multiple cats, with losses and new additions, over the years. Put the first two together when we got married, mine was 9 years old, his was 6, they finally agreed to disagree and just never both be in the same space at the same time. They would eat together, tho (food conquers all). Ten years later we lost the older one at 19, the 16 year old didn't seem to care, but they never interacted anyway. A year later we added a 9 year old. They agreed to co-exist peacefully. Two years later we lost the older one, and circumstances brought us a very young kitten a week later. The now 11 year old didn't mind at all for the first few months, but as the kitten got bigger, to about 6-7 months, she was making the older cat miserable, jumping out and play attacking her every time she moved. Big cat was slinking around the edges of the rooms, trying to stay under furniture for her own protection. So we got another kitten about 7-8 months old, hoping the kittens would play and leave the big cat alone. This worked, the two kittens never became good buddies (snuggle together type buddies) but since they were both so active, they played and chased together and left the old cat alone. A few years later the old cat passed on. The two younger ones were settled adults now, and did their own things, did not seem to miss the old one, but hadn't really done much together anyway.
A bunch of years later, we got a 6 month old kitten. One of our adult cats was fine and would interact, the other wanted nothing to do with her. A couple months later we let our teenager get her own 4 month old kitten. Eventually the two kittens made peace (surprisingly enough, even tho both came from many-kitten & cat situations, they freaked out at the sight of each other) and would play and hang out together. One of the older cats passed on from health problems, but as she hadn't liked anyone else at that point, not much noticeable change when she was gone, overall.
Then, after 2 years, the teenager took his cat to college. The young one left behind missed her terribly (both were about 2), and roamed the house wreaking havoc on our lives out of boredom. So we got another kitten. Took a few months for them to agree to like each other, and the old cat we still had was just fine with everyone, he just lives his life, whatever. A year or so later, the new kitten settled down but the other young cat is a very active hellion, so we got another kitten to occupy her when her other young buddy was asleep. Old cat said "whatever", younger cat became friends quickly, middle cat took a while but then accepted. Only now the youngest cat is a bit over a year, he's still wild and crazy, and mostly the others just tolerate him and his antics until he becomes too annoying. The 4 year old and the 2 year old will co-sleep and groom each other. The two year old and the 1 year old will co-sleep when the 1 year old is tired enough to stop moving. The 1 year old is kind of annoying to the 4 year old, still, she tries to groom him, he says okay, that feels good for about 10 seconds, then turns it into a wrestling match which then gets rough; I don't think they particularly like each other in general. The 15 year old mostly just sleeps in the sunny window, every now and then he plays briefly if someone offers, but mostly he just wants to eat and sleep, tho he's fine if someone else wants to join him sleeping in the big sunny window. He's the one who never objected to a new cat joining the household.
But even tho there was often no overt sorrow at the loss of another cat, in all cases we could see a change in behavior when someone passed on or left. Rarely has it been obvious mourning, but the dynamics in the house did change.
So...introducing a new kitten will probably annoy your older cats for the first year or two. Introducing an adult cat may work better, if personalities agree. Cats who were friendly and interactive with each other have seemed to miss their buddies, cats who simply co-existed or didn't like each other were less affected.
