How do I make my cat not afriaid of my new kitten!!!

We have had my cat Nahzy for 5 weeks- she is a spayed female, 1 year old. 1 week after we got her, we got a 6-month old kitten named Hayes. We immediately kept hayes in another room and never brought him out around Nahzy. Hayes has been in his own room for 1 month now - he is a neutered male. Nahzy has been very curious of Hayes since we got him; she likes to sit on the other side of his door and investigate, but no hissing or growling.
This weekend we let Hayes out into the rest of the house. Unfortunately, he thinks his big sister is a big playmate. He tried to play with her once and she hissed at him. Despite her hissing, Hayes doesn't seem to get that she doesn't want to play, and 5 minutes later, he tries again. Now, whenever we let Hayes out, Nazhy gets very tense and scared. She is twice his size but she is very timid of him.
Is there anything that I can do to reassure my older cat that there is nothing to be afraid of?
Also, is there anything I can do to get my kitten to realize that Nahzy just doesn't want to play with him yet?

Please help! I really want to keep them both and want this to work.

We recently got a new kitten and had to slowly introduce our two cats to each other. One thing that was suggested to me was to put a blanket in the new kitten's room for her (Penny) to sleep on and then once she slept on it to put it in with our other cat (Target) so he could get used to her smell. And to do vice versa so Penny could get used to Target's smell also. We would also let them smell each other in the same area of the house for a while too. Target was terrified of the new kitten for a while (and he was twice her size) but after a few weeks we could let them interact unsupervised. Target used to be a very unfriendly cat, but about a month after we got Penny he started getting more friendly and playful.

He's now just over a year old and she's about 6 months old, and they are the best of friends. The other day they were curled up napping together.

I hope this helps you some.