I took a quick look at a fansite blog, and the question of whether Yukiko has non Japanese heritage came up. No conclusive answer other than one poster had a vague friend of a friend attending a handshake event, and Yukiko mentioned there that her mother is Spanish. So for now this has to go into the "Risako Sugaya has a Canadian Grandfather file" the file of neither proved or disavowed familly heritage stories.
The more interesting discussions revolved around who she reminds people of- Yossi, Maki Horikita, Mika Todd, Mika Todd/Atsuko Maeda mix.....