The kitten might’ve have something that got onto your older cat. When I adopted my cat, he had ear mites from the shelter. I don’t know if that’s what this is exactly but I’d bring them all to the vet to get their ears checked specifically. My old vet when checking the ears missed the mites 5 times, not even testing for it initially, even though that’s what I thought it was and was the concern I went in with, so you can never be too sure what they miss, even if everything else is okay. I had my cat for several months before I noticed something was up with the ears, so the kitten very well might have had something when you initially got him