Okay, I get it's a manga trope, but why would you actually blurt out that someone is a little sister when they're in the process of informing you what their relationship is? It's just a strange and unattural thing to do. Maybe they're a cousin, maybe they're a big sister that is shorter. Maybe they're not even related and just happen to look similar.
This manga is generally pretty well written for what it is and this doesn't ruin it or anything, but that is just one of the oddest ways of having the 'WHAT? YOU MEAN YOU'RE HER MOTHER?" I think I've ever seen.