I just brought a new cat into the house myself. (Well, it's been about 3 months now.) This is a transition you always must go through, because cats are territorial and don't like new cats coming in. Hopefully it will make you more comfortable to know that this is normal. And the older the cat, sometimes the harder it is to accept a new one.
Fortunately my older cat spent a good part of his life with another cat, but he'd had 2 & 1/2 years by himself and got spoiled by it. But cats weren't new to him. He also is an extremely curious cat and his curiosity finally won out over his anger. But still, three months later there's sometimes some growling and hissing going on. The younger one usually is growling, while the older one stands guard over her, presumably establishing his predominance.
One added issue you have is a young kitten with a lot of energy, who will want to play, combined with a cat who probably is just sleeping a lot these days and may not want to play. The older one may just start playing eventually (and get some exercise). Or this may continue to be a problem, meaning spend time playing with the kitten and have plenty of toys, as the older one may not want bothered.
It's already been said not to leave them alone together, until you know they're OK together. But when you're there to supervise, let them do whatever they want, unless one could seriously get hurt. They need to establish their own territories and decide who is boss -- they will do this for themselves. So just ignore the hissing and growling, and only jump in if a serious injury could happen. New behaviors may happen, but older ones could come back. (Like my older cat always sits on the footstool of my recliner with me. When I got the new cat, she would jump up and sit on my lap, and the older cat wouldn't get on the chair at all, but sit on the floor and watch. But after several weeks, he finally started getting on the footstool again and is back to his normal behaviors.)
Give the older cat plenty of attention and affection, so that she knows she is still loved. You can't help but give the kitten attention also, but she needs to know you still love her.