In the beginning, she supports her sister's romantic feelings towards Tomoya, repressing her own feelings in the process. When a rumor spread about Tomoya being a two-timer between her and Ryou, she distances herself from them to ease the tension.
Who Tomoya really chooses in the end is up for debate.
This is Saya, the cosplayer behind Gintama's Yagyu Kyubei and Sound Horizon's Violette. Quite an impressive cosplayer, isn't she? Thanks to ~MPzero~ for the beautiful photography!