The 48-year-old man, identified only as Detlef S., "has admitted to all the acts he is accused of," said Alexander Walter, a spokesman for the court in Koblenz, western Germany.
Previously, Detlef S. had confessed to abusing his daughter and fathering the eight children with his stepdaughter - one of whom died - but denied raping her.
Prosecutors have called for him to spend 14 years and six months behind bars for his alleged crimes.
In his closing arguments, prosecutor Thorsten Kahl told the court Detlef S. had begun to abuse his daughter when she was nine years old and his stepdaughter when she was 12. His defence lawyers argued the sentence should be reduced to nine years and six months.
A verdict in the trial, which has shocked Germany, was expected Tuesday.
In total, Detlef S. is charged with 350 counts of sexual assault and grievous sexual assault of minors over a 20-year period in the tiny village of Fluterschen, near Koblenz.
He is also accused of prostituting his stepdaughter and his biological daughter out to others while he allegedly watched.
Several German media outlets have compared the case to that of Austrian Josef Fritzl, who held his daughter Elisabeth as a sex slave in a cramped dungeon for 24 years. He was jailed for life in 2009.