Answer:

The behavior Kisses is displaying is very common in that situation. She is mourning the loss of her brother and is confused by his disappearance. It's not unusual that she's carrying around Cupcake's favorite toy as a way of comforting herself. Her cries are normal warning sounds that cats make when they have a prized "prey" in their mouth. They want to warn other cats in the area not to try to take it away.

You may find that Kisses starts spending time in Cupcake's favorite napping areas. This is normal as well. You can help her through this difficult period by planning lots of play sessions with her for distraction. The playtime will also help her realize that not everything in her world has turned upside down.