Jessesgirl, in my experience some cats are much more territorial than others. From what you’ve indicated, Sienna is more territorial than Gabe. In addition, some cats handle changes, in general, better than others (not quite the same thing as territory, but it can sometimes be related). So it may be that Gabe just isn’t too bothered by the kitten, it’s just different and he’s handling the adjustment well. Sienna, however, is definitely a bit more put out, and will probably take more time to be comfortable around the kitten.
(What’s the kitten’s name?)
Most striking instance of this was when I adopted two cats from the animal shelter in Chicago. They were unrelated, but both female brown tabbies; one longhaired adult, and one shorthaired 18 week old kitten.
I let the kitten (Mal) out of her carrier first. She was visibly nervous and a bit spooked, and started slinking along the wall, sniffing as she went. I waited a couple more minutes, and then I let the cat (Lady) out of her carrier.
She looked around calmly, sniffed a couple of times, and then proceeded to stroll around the apartment to acquaint herself. She acted as if she belonged, right from the get-go. She saw Mal, and calmly sat down and looked at her.
Mal completely freaked out! She hissed and spat and fuzzed up every hair on end; she backed away from Lady until she was at least 6 feet away. And that’s how it was for about 2 weeks. Every time Mal saw Lady, she hissed and growled and got her knickers in a twist. Lady never returned the favor; she just watched.
Finally, after a couple of weeks, Mal stopped hissing, and they grew to be best of buddies. They eventually slept curled up with one another, to the point where it was hard to tell where one cat stopped, and the other began.