Because Nino is a bit biased and can't read her sisters' minds to corroborate her claims. It's in her nature to be bold and go after what she wants unapologetically, but that's not really love. I haven't yet seen her approach Fuutarou romantically in a way that isn't selfish. Yes, she's shown her love for her sisters throughout the whole damn manga, but Fuutarou? She still hasn't taken his feelings into account. She's still demanding, still abrasive, and still quick to anger when it comes to him. Flirting with him at work for one chapter seems to have been turned into some grand love story by her fans.