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.
I foster kittens and have 2 cats of my own, so I've done introductions MANY times, and introducing them face to face so quickly is a very risky way to go about things, but luckily, it doesn't seem to be a total disaster for your cats. I highly recommend checking out Jackson Galaxy's guide to cat introductions to make sure things continue smoothly.
This is part 1 of a 5 part series. The subsequent parts are linked within: https://www.jacksongalaxy.com/blogs/news/cat-introductions-part-1-before-the-introduction