So, what was the point of him throwing away the paper then? Is he not willing to admit that he thinks of his sister as his most precious treasure? He doesn't strike me as the "tsundere" type, he was willing to take frequent beatings from the jealous perverts (why do they have to be this much of a thing?) just to keep his sister happy, and he hangs out with her outside of school all the time too, so he doesn't really have much to hide from her there.
The way tsudayama appeared was pretty suspicious too. It implied that he left the three spergy dudes he came with outside as he walked in by himself (to get a drink/takeout? for the rest of them?), listened in onto their conversation ("about what you were talking about before"), then dropped in just to tell her that his brother has no feelings for her (which is true) and that she should stop thinking of it as anything more (or some other motive?), but I might be reading into this a bit too much for this one here. It just felt like the sort of coincidence that leads in to some conspiracy involving the ally turning out to be evil or something.