Insecure isnt the right word. When Kyou was in Tomoya's class she'd go home and rave on about how cool Tomoya was to her sister. This gave her sister a good first impression and repeated bouts kept reinforcing this message that Tomoya is a good guy. The following year Ryou and Tomoya are placed in the same class and with such a good impression of him, Ryou confirms her feelings and tells Kyou. This takes Kyou aback, who, then feels obligated as the older sister to see to her younger shyer sister's happiness and her success with Tomoya. She puts her own feelings aside, but soon realizes they are too strong to ignore and agonizes over what she has done and how she's played things out. In other words, she was confused and tried to do what was best.