Lots of hugs and thoughts for you in the loss of your sweet kitties. You may be grateful that they had very long, very happy lives with someone who loved them so much. I agree with what was said earlier that your 12 year old probably missed her friend, and that her time came quickly because of it.
Please try not to feel guilty for your new little boy. You should never feel guilty for rescuing and loving a needy kitten. Your other cats know you don't love them any less because of it. They know what a wonderful Mummy you are and that this little man will have a long, happy life the same as they did.
It can be hard to get used to a new arrival, especially after the passing of two such dear and long-time friends for you. You may have difficulty adjusting because of your confusing feelings and your grief, and this little man is new and not what you're used to.
I suggest that you give both of you time - time to get used to each other, time to get used to the newness of the situation, to get to know each other and to fall in love with each other. You will miss your kitties for a long time to come, but know in your heart that you have enough love for a wealth of kitties, and that your beloved babies are in a happy place and looking down at their new little brother getting the wonderful life he deserves.