Okay, they're not biological sisters, but I still like to think of them as sisters. Anyway, most of the time they get along, but once in a while my younger cat will run after my older, more timid cat and attack her (probably wanting to play), but it scares my older cat, who hisses and hides for a while after being attacked. I don't want to yell at her, but I don't know what else to do since it's stressing my older cat out. Any advice?