Roll one's eye, as if

Back in the third and fourth grades she used to clown around with her brothers in the branches or peel big chunks of bark off so they could slide down the crook in its trunk. It seemed like they were playing in it whenever my mom took us somewhere in the car. Juli'd be swinging from the branches, ready to fall and break every bone in her body, while we were waiting at the stoplight, and my mom would shake her head and say, “Don't you ever climb that tree like that, do you hear me, Bryce? I never want to see you doing that! You either, Lynetta. That is much too dangerous.”

My sister would roll her eyes and say, “As if,” while I'd slump beneath the window and pray for the light to change before Juli squealed my name for the world to hear.

What's the meaning of the underlined sentence?