Don't worry Ruka! Your Ryuu-nii already knew you loved him! ...as a sister.
But don't worry Ruka! Your Ryuu-nii also loves you! ...as a sister.
Kinda the issue with having feelings for the cousin you grew up with isn't it? She is just so doomed from the get go. My prediction is that he will decline, hint at at liking someone else romantically while also making she that he views her as a little sister. Which Senko will probably overhear and panic over in some overly dramatic Senko-like way.