I have two female one year old domestic short hairs. They are sisters from the same litter and have never been separated. Overall they are great friends and rarely fight. One cat Sookie, is extremely playful and the other, Pumpkin, not so much. When Pumpkin does play, I noticed that if Sookie jumps in, Pumpkin will sit back and no longer play and I can't convince her otherwise. Is this an alpha situation or something else? Any advice?