Just a year out of elementary school, Ishikawa had accepted an invitation to play at the 70 million yen ($775,600) event in Kagoshima, southern Japan, organizers said Wednesday.
Ishikawa quickly declared she wanted to follow in her multi-millionaire brother's footsteps and turn professional after some initial butterflies over entering the field.
"I didn't know whether to accept (the invitation)," she told Japanese media a day before Thursday's opening round. "I was so nervous about whether it was okay for me to play.
"My target will just be to avoid making a lot of bogeys. I will concentrate on my driving. I don't want to shoot an 80. I've begun to really feel I want to turn pro."
Ryo, who in 2007 became the youngest player to win on the Japanese men's tour as a 15-year-old, offered some sage advice to help soothe his little sister's nerves.
"Dad used to tell me just to let rip with the driver when I was young and tell me it was too early to worry about my score," the 18-year-old told the Nikkan Sports.
"He'll be saying the same thing to Yoko. According to our mum, dad is pretty soft on her."
(Reporting by Alastair Himmer; Editing by John O'Brien)