To the other cats, does it seem like you are focussing on the new kit more? Giving it more attention, etc.? Cats (and even dogs, or even humans for that matter) can find it offensive if the newcomer gets more attention than the incumbents. Think about it from the animals perspective - you did not rescue a kitten (though you did), you brought a stranger home (to compete for resources - food/warmth/human-attention, etc.).
The kit has been adopted by you alright but it has not yet been accepted by the cat-pack. It is yet to find its place in the hierarchy (yes cats have it too, though it looks a bit different from dogs). When you prioritize the kitten, to the other cats it seems you have installed a new top-cat.
Here's how I would handle it (done it in the past): take care of all the physical needs of the new kitten but ignore it beyond that, pet/hold/whatever your older cats. If there is to be a cat in your lap, it has to be one of the older cats. If the kit is to be in your bed, it must first be allowed/accepted by the other cat that already sleeps in your bed. If there is a conflict, the older cats are always right. Do you understand?
If you want things to go back to normal, your present behavior has to seem normal to the cats (compared to your past behavior, when the kitten wasn't there). The kit has to come last in line for human-affection/attention (or find itself a new home if it cannot bear being at the bottom of the rung). The older cats have to see you as being on their side, if you want them to stay.
Let the other cats also accept the kit, over a period time. As part of this process, there will be hissing, etc. (possibly even pooping to mark territory). The newcomer has to work its way into the pack, and should be discouraged from undesirable behavior like pooping/peeing to mark its territory.
If you force a new order (with the new kit at the top) upon the older cats, they will naturally find it offensive, and look for other places to be (treating you as just a food source) - as is already happening.
The problem will be a bit bad in your case, because the older cats are related/family and the new kit is not part of that same family-group. It is not unwise to nurse the kit back to health (keeping in mind the inter-cat-dynamics I mentioned earlier) and then let someone else adopt it. You can keep it, but it will need a bit of careful effort and observation of animal-behavior/psychology on your part. Good luck!
