The man, now 46, was 25 when he first had sex with the 12-year-old daughter of his then-wife in 1996.
Over the next four years they were intimate hundreds of times, according to the prosecution at his trial in Brugg.
In 2005 the man divorced his wife and moved in with his step-daughter. Married in 2011, the couple now have two children aged five and six.
Interviewed by newspaper Blick, the woman, now 29, said: “He’s my confidant. I never found the love that I sought from my family.”
The defendant said: “I love her, but I knew that I was going to have problems because of that.”
The couple’s situation was brought to light in 2013 after an investigation into the man’s use of child pornography.
On Tuesday the Brugg court pronounced the man’s actions with his lover to be “a serious crime” even though the two are now married.
The man received a sentence of two years in prison, suspended in favour of therapy.
“I know that I have to work on myself,” he said.
The couple are now fighting to keep custody of their two daughters.
Speaking to Blick, the man said: “I am not a threat to them. I would never do them harm. I would never touch a child.”
Asked about the start of his affair with his then step-daughter, he said: “I knew it was forbidden, but you can’t help who you fall in love with.”