When I first brought Wylie home last year, he was scared of my older cat. Kedi wanted to be buds right away, but Wylie would hiss, swat, etc. , also hid under the bed alot!
It took about a week, then they were fine.
With different cats, the time will probably vary. I also think the fact that there are 2 kittens with 1 older cat can have an effect:the kittens have each other, so they don't HAVE to get along as quickly with Muffin.