Boy really is so focused on Shia, and the surface appearance of his brother's actions, he doesn't realize he's actually trying to do him favors and maybe even be a bit of a wingman.
Tsumugu made a dumb excuse, but he wanted to help Tasuku get out of an obvious rut by getting him out of the house, then he noticed his bro might be hiding a book (assumes the wrong kind, but still) so gives him some other books to have a better time hiding it, with another excuse to step away. Lastly, I suspect he also noticed Shia hiding that book, so spooked her to help him notice what it was.
Jury's still out on the sister.