I think it is the personalities more than whether they are related or differ in age. Was the older cat an “only” cat? If so, it might be hesitant about sharing or get jealous. Is there a littermate in particular that your kitten seemed to get on well with? Do you have the opportunity to introduce the two cats? (Although you cannot always tell, they might start hating and wind up loving.) Unlike dogs who crave companionship, once they are past kittenhood, most cats just do their own thing!
That’s good advice. Thanks very much. We have to talk more with the ladies who actually know all of these cats. That will happen tomorrow.
-JT