I brought home my kitten two years ago and my older female still doesn't like him. Now, she's not stressed out, but she hisses, growls and screams at him when he tries to play with her. At first I noticed she seemed to be depressed, just like you are describing with Stanley. But from the beginning I developed a routine (after the initial separation period). Each morning when I get up Jax, the kitten, goes into "his room" and Paizly gets the rest of the house with me. I also go into Jax's room and spend some time with him alone too. Then they stay separate until my boyfriend gets up a couple hours later and he lets Jax out. That gives Paizly time without having to deal with the annoying little brother.
Sometimes, (like this morning at 2am!!) he just won't leave her alone so he goes into his room just so we can sleep. It might sound mean, but he has everything he could possibly want in that room. Food, litter box, cat tree at the window and TONS of toys!! And his very own twin size human bed to sleep on. He never cries when he's in there either.
So please give it some more time. And make sure to spend quality time with Stanley without Winnie around. Also spend time with Winnie playing and getting her tired out so she won't bug Stanley so much.
Good luck!!