How can i help solve this problem?

Alrighty so me and my bro have this cat thats been in my family for probably 4 years now and the sad fact is hes lived all this time completely away from other cat companionship. weve recently tried bringing another newborn kitten into the mix but that didnt work out well. needless to say he felt very threatened and so that stopped. Its fair to add that this cat is a predator, hes mastered the woods around our house and regularly brings us birds and other things he hunts. So the other week this old lady came to my brother and basically begged him to take these two 3 month old kittens. they are the cutest cats imaginable theyre brothers and they even act like it, they travel with eachother all the time and always play with eachother. Theyre in their prime so theyre not technically babies and can take care of themselves but were scared of leaving them with the other cat so were rotating them in and out of the house so they dont see eachother. The big cat that we already have already tried to kill one of them right off the bat when he saw him the other day so we are now really scared of what to do with these cats. The biggest thing is we dont want to get rid of them and were willing to do what it takes. But i have to ask now what can we do to psyhcologically adjust the cat to these new kittens. i know theyres some type of dog whisperer trick out there so please help if you can