Shirou gives Sakura the ability to change to how she would have looked as a Tohsaka.

Sakura Matou can change her appearance to what she would have looked like had she not been given to the Matous.

Sakura watched as Shirou wrote that down. How does that wor... she thought before it suddenly came to her.

Putting the pencil down, Shirou looked up at Sakura. As he did, a wave of dark brown, almost black, color spread through her mane. It was almost like a ripple in a still pool water. Sakura's eyes were also transforming. The color of the iris shifted from her usual beautiful purple to an equally lovely aqua.

They really do look like sisters, Shirou thought in wonder as he gazed at his girlfriend's transformation.

After a moment, the change was done. Sakura looked over her hands, to see if there was any change. She noticed her hair, now darker. She scurried off to the bathroom to look at herself in the mirror.