Even Though I Reincarnated as a Supporting Character, Is It Okay to 'Capture' My Stepsister?

I Can't Wait for the Protagonist to Show Up! The Supporting Character Brother Saves His Step-Sister Heroine! Before I knew it, I had reincarnated as Kanzaki Eiji, a side character in the game 'Koikoi ni Somaru Sora'. Eiji is the stepbrother of the game's heroine you are trying "capture". In other words, there's supposed to be another protagonist. So, I figured I should enjoy my second life without stirring up too much trouble. But—my stepsister Suzuna, the heroine the protagonist is supposed to "capture", seems to be in a lot of pain. In the game, the protagonist saves Suzuna three years from now. Should I just leave her alone until then...? No, that’s impossible! Even if I'm not the protagonist, as her brother, I will find a way to save Suzuna! The struggle of a supporting character brother to save his heroine sister begins!