Oh that's so upsetting and stressful. I know.I've been there.
But...try very hard not to give up on the new cat/kitten.I think this could be just a passing thing.These things happen..our cats/kittens get better and in a few months/year,it's all forgotten.And you realize how much you love the one that got the others temporarally sick at one time...way back when
They both may need antibiotics.But it will soon be gone after their both taken care of.
We all share each others sicknessess,except the ones that kill.We keep those cats seperated.
So try not to worry.
And best of luck and prayers for all to be healthy very soon.