If he was being honest, and Christian had a tendency toward the kind of obnoxious honesty that arose from never holding anything back, he would say that he was a bit irritated at having to trek all over Diagon Alley with Selwyn.
He didn't dislike being in the Wizarding shopping centre, mind. Anything that got him away from the backwater they called a home was brilliant as far as Christian was concerned. And it wasn't exactly that he disliked Selwyn being there. More the way he'd been fobbed off onto his brother like an old shirt their dad didn't know what to do with. "Oh here, Selwyn, you'll get better use out of this than me." He could practically hear the words.
So far, he'd done all he could to show his displeasure, which was mostly dragging his feet, speaking in monosyllables and refusing to help in any way, shape or form. And, contrary to the way he'd assumed his brother would react (in a spectacularly explosive fashion), Selwyn had barely even noticed.
In fact, right now, he was up the front of the apothecary organising Christian's potions kit for next year while Christian himself picked up almost everything in the shop. Except, of course, the things with warning labels because he might be annoyed but he wasn't stupid. He rolled back his sleeve and plunged his hand into a vat of snake eyes. Slimy.