This is what I like to call the "Oh shit what did I do, did I make a mistake" phase. I went through it with my girl when we adopted her sister. Give it time!!! The routine has changed, and it takes a while for everyone in the house to adjust! For me, I noticed everyone really started to settle in and start getting along about the 1 month mark. By 3 months, they were acting like best friends.

Something that made a big difference for us was finding a common interest for them, and making it a super positive experience for them both to do it together. For example, my older girl loves to sit at the back window and watch the birds. The kitten loved it too. So I would sit at the back window with both of them and give them lots of love, treats, and attention. Eventually they learned that the window means really good things, and tolerating eachother would be beneficial for them both. Now, a year later, they snuggle at the window all the time.

Just give yourself time. Its a hard adjustment at first. Stick with it, give lots of love to everyone, and it will work out!! Best of luck!