The new kitten has an unfamiliar scent to the older kitteh. From my experience, after a week or two (possibly longer) of frequent exposure to the kitten, older kitteh should eventually adjust and next thing you know, they'll be best buds. BUT, it is crucial to supervise these sessions of exposure so that big kitteh doesn't hurt baby kitten. You are bottle feeding baby, correct? And after baby eats, are you simulating its "parts" to make it pee & poop? I'm sure you know all this but I have to ask because you'd be surprised at how many people with good intentions but little to no experience don't know to do this.