YOUR hips, as the rider, control the horses hips. So first ensure your hips are straight (and therefore so are your legs). If your hips and legs are not straight than chances are the horses hips are also not straight. Straight on a curved line is simply - the smaller the curved line (circle or arc of a circle) the more the riders body will be curved in order to be straight - confusing isn't it?
Think of a small circle with you on your hands and knees. Now place your knees with the arc of the circle in the center of the gap between your knees. Now do the same thing with your hands.
Where is your waist? If you are straight the center of your waist (belly button area) is on the center of the circle (arc).
Rider controls horses hips with their hips, horses shoulders with their shoulders (and reins) and wasit with their legs at the girth (horse should use riders legs at the girth to "bend" around - so they bend like a bananna not a candy cane.