Just brought home a new baby brother for my 3.5 year old adopted at 10 months cat, Autumn. I wanted to give her a sibling she can play with. I got both cats from the same adoption shelter. Autumn is very hesitant and hisses at Dewey but seems pretty tolerant already with this photo taken only a few hours after bringing him home. Dewey got up there all by himself and very much wants to play with Autumn. With some more time and introduction therapy I have a feeling she’ll come around pretty quickly.
Read up on it, there is a lot of information on how to do it properly. Just keep in mind it can take a long time for the new kitty to be accepted. I added another one to the family a few years ago and she was tolerated for a year and now they’ll snuggle and play together…not as much as the kitten would like but sometimes.