From Sayu's gree blog, courtesy of Maachan.
http://gree.jp/michishige_sayumi/blog/entry/664816704 Except Sayu, apparently everyone in current Morning Musume is the eldest daughter of their respective families.
After a little bit of digging in 2ch, MM members' siblings
Sayu : Older brother, Older sister
Reina : Younger brother
Fukuhime : Older brother, Younger brother
Eripon : Younger brother
Riho : Younger sister, Younger brother
Zukki : Younger sister
Harunan : Younger sister x2
Ayumin : Older brother
Maachan : Younger sister x2
Haru : Younger brother x2
Sakura : Older brother, Younger sister
Sayu and Ayumin are also the only ones who are the youngest child in current Morning Musume. That would explain some things I guess
edit- 6/4/13
On Michishige Eleven concert in Hiroshima Sayu and Reina talked about their academic background
Reina is a middle school graduate, Sayu is a high school graduate in a correspondence school.