Don't like that trope when it's unexplained. Without a reason, I automatically assume it was society which forcefully changed them into it's ideal image. Not a fan of that idea AT ALL.
The reason here could be simply for Tsukihi to not recognize her, I don't think there's anything deeper than this plot device reason, but if you want an in-universe reason, it could be because Kanade wanted to have a beautiful body for Tsukihi to enjoy.