This is something I've found quite useful for establishing a playful, flirty dynamic with a girl. The "little sister" mentality (as I call it) is something that can be used to tease, neg, and engage in witty banter with a girl. The mentality is simple: if this girl was your little sister, how would you treat her?

Answer: you would tease her! You wouldn't act all serious and logical and try to "figure her out." You wouldn't engage in hour-long, deep philosophical discussions about the meaning of life and the physical makeup of the universe. Keep things simple and fun!

Using this mindset really creates an aura of "aww, you're so stupid, but so adorable" between you and her. It can be used to build a bit of rapport and sexual tension, and helps ensure that you won't be labeled "the nice guy." Have fun with it.