Just one more thing to add to all the good advice you have received already. Sit with Misty and talk to her as you both watch Abby play by herself. If Abby approaches during this special time with Misty, ignore her and focus on Misty. You want Misty to know that she is the most important cat to you (until she has accepted Abby), and that you won't allow Abby to rob her of your attention. It's so easy to focus on the new and younger arrival, but your older cat is watching you and assessing your response to the new cat all the time. I know this approach has helped ease a new cat into my household many times.