Just got a new cat, and my old one isn't very happy?

Shower Angel with more attention, treats, esp. if she comes when you call her, and a new toy with catnip in it. Pay atention to Poof too but also "talk to them" and say how they are Sisters, and for Angel to show her the rules of the house. I did that with mine many years ago and it worked to talk to them.

It might take a while but yes, they will eventually be friendly to eachother.

Look from angel's perspective.

She's been living with you for several years and then suddently this obnoxious kitten appears in the house and she's never see her before. Remember, this is a cat. She will have to get used to Poof being around before they'll be friends.

Once Angel realizes that Poof is nice, they might get along. Otherwise they might never get along but also don't try to make them get along, they will just end up hurting you or hating each other more. Hope this helps, Good luck!

The first few days are the worst. Suck up to Angel just in case she thinks she's getting replaced or something. The first day mine were together the new one slapped her new housemate in the face but they're laying on the bed together now having a nice kitty nap.

It's been my experience that leaving animals alone is the way for them to work things out. Just make sure you don't show more affection or attention to the new kitten. In fact, you might want to baby your older cat for a while.