Police identified the 23-year-old woman as Madeleine W., and said on Wednesday she could have been alive when she was thrown into the concrete in a pit more than a metre deep.
They said they were certain she had suffocated, either before or after being buried in Essen, North-Rhine Westphalia.
Her stepfather, 47, and half-brother, 21, have been arrested on suspicion of murder and are in custody.
Police murder commissioner Eckhard Harms said a “serious breakdown in the family” could be the motive for the alleged murder.
The stepfather from Essen has been under investigation for more than a year on suspicion of sexually abusing his stepdaughter, prosecutors said.
It is believed he is the father of his stepdaughter’s two-year-old child.
The mother disappeared last Tuesday after dropping her daughter off at the local kindergarten.
The body was found after police noticed freshly dug ground around a gazebo in the garden. She was buried more than one metre in the ground and covered with two layers of concrete and earth.