Just remember only one day ago the fmc actually showed him her boobs, so it's fresh in his memory. Bro probably burned the sight of them into his brain in the bath scene too. Also I think the sister's boobs are a bit smaller than the fmc, and I believe she has a distinct beauty mark on her right nip, so everything is there for him to figure out it's not her. The question is how far will it go before he figures it out. Oh well, 2 weeks until we find out.