Crosley was 4 1/2 when we got Wrigley and I was super worried about how she would react. I can remember feeling guilt on the drive home from picking up Wrigley. I thought Crosley would hate us. She didn't even acknowledge her for a day or two. Wrigley was a 12 week old energetic puppy and basically kept bugging her big sister until she had to pay attention!
They lay together a lot and Wrigley is stressed if she's not here. It's funny because Crosley is like 17-18lbs and Wrigley is 6lbs, but Wrigley is the boss and tries her best to keep her in line! She's a tattle tale and has a snotty/brattiness about her.
Overall it was much easier then I expected! I really do think they like having the company.
Two to three was more difficult, but there were a lot of factors in our going from 2 to 3 though.