We have 3 sibling cats that have been together since birth and have always been best friends == 1 male and 2 female, spayed, neutered and declawed front only and they are indoor cats. We go on mini trips often and leave them for up to 4 days on their own and have had no behavioral problems ever. We went away overnight for Thanksgiving and upon return observed aggressive behavior by the smaller female towards the male (catfight). This catfight is now 3 days running and not resolving itself. We think maybe a stray cat is showing up and confronting them visually through the sliding glass door and causing the outbursts but we are not positive. What could cause such a drastic change in behavior in the smaller female being hostile to her brother. Any ideas??????????