I'm not that familiar with the Watatsuki sisters, but Yorihime sure has a pretty outfit! From the pictures on the sides, it looks like the dress is actually white, but I guess the lighting or something in the middle picture makes it look somewhat purple.

At first, I thought that was a wedding dress... but it doesn’t look similar to the ones that the other characters have (aka, the rebirth costumes), soo... what kind of dress is yori wearing?