I guess this... Ry'leh or whatever, looking upon you has nothing to do with sexual intercourse or desire or anything like that, if the brother could see it as well. If the mystery eyes do inhibit some kind of fear within the children, perhaps he's "strong" enough to resist any ill effects that might be received from being observed. Or perhaps he's even afraid, but still enjoying the feelings of fear, which would be kinda creepy I suppose.
Personally I'm not a fan of incest, but I find the story interesting enough to not let that detail deter me from continuing this. At the very least, Tooru isn't some horny bastard lusting after his sister(at least not yet), though I don't get that same "it feels nice to be with my sibling like this" feeling out of Yume, who seems to be more conscious of the sexual implications of the situation =02.