Hissing concern

Hi all! So we have 3 cats in the household, all female, 3 years old, 9 months old, and 7 months old. We’ve had the 3 year old for over a year and we adopted the kittens together 4 months ago.

Our older cat still hisses at the kittens daily. She doesn’t fight them or anything, nothing physically concerning. I think she does it when she just wants to be left alone tbh. I guess I’m just concerned because of how long it’s been and I’m not sure if it’s something I should be concerned about. Is it just her establishing dominance? Should I just let them be? Is there something I can do to make them more comfortable? Any advice is appreciated!