It sounds like your 8 yr old got the upper respiratory infection from your kitten. I don't think it's a poisoning, otherwise your vet would've done more than just give an antinausea medicine.
Your 8 yr old needs antibiotics, too. Apparently, she is affected worse than the kitten is. It sounds like she's having a lot of trouble breathing, too. You need to rush your cat to the vet, this is NOT her stomach.
What medication was your kitten given for those ear mites? Have you tried calling the vet again? They need to know your cat isn't improving. You could also try an emergency vet. I doubt that a vet clinic is open this late.
What a tragedy. I am so sorry. I hope your kitty feels better tomorrow. Please let us know how she is doing.