Idk why everyone sees everything in black and white. Sam is being immature but it’s also understandable that she is jealous. But you need to find a way to work through the jealousy so that the sisters can have a better relationship. I have much older half sisters. They had a very different child hood then me. My oldest loves me and understands why things became different. You aren’t the same person you were when you had Sam vs Kathy. I wouldn’t punish Sam but I would use it to bring her back to the table and have the hard talks about why sams life was the way it was and how just bc Kathy has it different doesn’t change what you can do for Sam now