Newly adopted approximately 1.5 year old female to household with 12.5 -year-old male.

I have a 12.5 year old male tabby (Kyle) that I adopted with his sister (Maxine) when they were kittens. Maxine died last July of cancer. I had a very close relationship with her, was devastated at her loss and have just now been able to think about adopting another female, realizing that it might be difficult to bring another cat into the household with Kyle. While passing through Petco pet adoptions on a Saturday two weeks ago, I spotted a female (April) whose appearance was similar to Maxine's. She had just recently had a litter, was rescued from the shelter by our local rescue group and was spayed. April is a small cat, approximately 6-7 pounds. Kyle is 11-12 pounds, very affectionate and not aggressive. My concern was for the new female's safety coming into Kyle's environment. To make a long story short, although Kyle growled and hissed a lot, that's all he did, and he eventually was able to get close to April. April, on the other hand, is an aggressive, dominant female, who has just about restricted Kyle from every one of his familiar spots in the house. I love April and want to keep her, but I have to lock her up at night and when I leave so that she does not terrorize Kyle, who turns tail and runs whenever she "charges" him. I've sprayed water, I've done time-outs in the cage for April. I've put both in a room, each in a cage facing each other, for an hour or so. While they have (out of the cages), under my supervision, appeared to co-exist peacefully, lying on the bed together with me, in the same room together with me for extended periods of time, just when I think we're making progress, April always goes back to attack mode on Kyle (usually unexpectedly) and when he runs, she gets even more crazy and flings herself like a whirling dirvish at him. He's miserable, I'm miserable, and April is spending a lot of time-outs in a separate room. It's been two weeks, and while I want April to stay, I love and feel the need to protect my Kyle. I'm not ready to throw in the towel, but if things don't change soon, I may have to give up on this adoption. Any clues as to what I might try. April shows absolutely no aggression/ hostility toward people, is very affectionate. It almost seems as though she has multiple personalities. Is it possible she's mentally disturbed, or is her behavior perhaps a result of her background outdoors (perhaps feral ?) versus Kyle's protective environment since kittenhood in one home? Help please.