I think since tigris was like a character that hated the hunger games and she probably wasn't very happy when coryo became president a nd kept the hunger games still going. I don't even think she was very happy about how he runs panem and that's why she decided that she wasn't like his cousin anymore and she was ashamed of being his sister. She probably just wanted to do the least she could to help the rebels since she was with then since the beginning of the games. These are juts my thoughts. I don't think included any spoilers. :)
