I don't have any advice for your problem, but I do have a story that my help ease your mind. About three years ago, we had three outside cats. Leo-black and white dominate male and for the most part firendly, Stanley-
a orange tabby male-very affectionate, and a black and white cate we called Bandit the black came around her eyes. She was a female but we didn't know it at the time because she would never let any get near her. One day four kittens popped up in our shed-apparenty Bandit's and Leo's. Since they were in the shed, we had to go in there a lot for various items. When Bandit realized we had been around her kittens, she moved them then dissapeared and we never saw her again. The only problem was the kittens were only five weekd old-not big enough to survive on their own. My mom and I went to the pet store and got some stuff to take care of them. Then something really strange happened. I guess Leo realized that Bandit wasn't coming back, he started to play mommy. Not in the truest meaning of the word, but if one would runn off he would go get him and bring him back by the scruff of his neck. And when ever we were feeding them he would be right there, almost making sure we were doing it right. And for five weeks he stayed right by the garage(where we kept them). He and Stanley had always butted heads but it got worse between them, if Stanley got close to the kittens, Leo would chase him away-far away. I know it sounds crazy, but it's true. So maybe there's hope for your cats that the older one will maybe take the younger one under he wing (or paw)! GOOD LUCK.