....That is not the case in the slightest.
She's one year younger than Ren, and the exact same age Rise and Naoto were in P4. I don't see why people can declare them waifus and be so up in arms about Futaba. Or is it that 15 year old waifus are fine as long as they have big breasts?
They're not related in any way, shape or form. They might have a sibling relationship if you so choose but keep in mind....they've known each other for less than a week when this scene occurs in the anime and they are basically together for less than six months in the game. Contrast that with Nanako, who is Yu's cousin and Yu knows from the beginning of the game onwards.