Kittens 1 month age difference?

The older cat is probably jealous of the newer kitten. What you should ALWAYS do when getting another new pet, is keep it in a separate room for AT LEAST 2 weeks so it can adjust to it's new home. Then, everyday for about 20 min or less, take the older cat into the room, hold the new kitten in the space between your legs, (in a cross-legged position) and let the cat just be in the room with the new kitten, to learn to accept the kitten. If the cats gets aggressive you'll be there to stop the behavior. Hope that helps!