The older brother is doing the whole "perfect boy who is secretly creepy" schtick but it's in a context where he doesn't know these people personally or have any reputation with them, so it's just a guy being overtly creepy and somehow not managing to remember which leg he is lying about, an extremely basic fact to forget while talking condescendingly to everyone else like he's a total genius instead of bugfuck nuts with nonsensical logic. Him being bonkers isn't an issue, the framing of scenes trying to present him like a master manipulator for a plan any middle schooler could easily follow is just weird.