For years, Makiko Dazai had nagging questions about her sister's death from ovarian cancer in 2008.

Dazai, a 46-year-old housewife from Kiyose, western Tokyo, said her sister had been taking great care of herself, eating healthily and undergoing regular health checkups. She was diagnosed with ovarian cancer in 2004.

"My sister would choose to eat low-pesticide vegetables and brown rice," Dazai recalled of her sister, who was a year older than herself. "She would only drink a little occasionally and didn't smoke. If she had developed cancer because of her lifestyle, I thought everyone would."