I'm not gonna spoil anything for you other than saying he has zero sis-con/perverse interests in his sister.
I'm honestly surprised you haven't picked up on one of Yukito's particular traits.
I mean her brother certainly does seem overprotective at the moment and while I wouldn't say he has a sexual attachment so far, that's often where this strong attachment eventually leads to, in anime anyway.
Yukito seems lonely and his positive demeanor is likely a facade. I do not think he's interested in Sakura for example, but his closeness to her still reaches eyebrow-raising levels and unfortunately plays into her crush on him, even if he has no malicious intent. I mean she was just about to confess her love in this episode, so he dun fucked up. He's the older, smarter guy. Does he not realize any of this? Like, has he not noticed the most obvious infatuation of Sakura? Has Toya not told him about it? Why is this going on for 18 episodes with no end in sight?
His sis-con remarks didn't seem serious to me but, man, if he keeps repeating it and Toya can't even reply with a "stop joking man!" then I eventually start to wonder if there's more meaning to it.
I'll spoil myself on the plot because I no longer care anyways.