What's behind them, a window or a painting?

I made this painting from a photo I took of my sister and niece at the Art Institute of Chicago. As a youngster, I spent many hours at the AIC trying to develop my eye and find my place as an artist. The painting in the background is Sky Above Clouds by Georgia O'Keefe. After I completed this painting, my sister was thrilled because this O'Keefe is her favorite painting at the AIC! 

My sister has always been the softer one. I love how this came through in the postures, colors and textures of this painting.