Ruka is kinda screwed.
She can say ''I love you Ryou-nii'' however much she wants but the only response she will get is ''of course we are like siblings afterall!''. Like he views her as a cute little sister since they are cousins and grew up together, zero interest in incest there. Meanwhile he is practically head over heels in love with Senko and it is pretty much mutual. And while she is going on about how much she loves Takamura she can't stop talking about Sugita's big mountain so she got pretty much no chance of convincing Takamura that it isn't familial love she is talking about.
Also hopes Sugita and Risa gets together, Ruka can find plenty of big dicks in other places.