"They lived in the same neighborhood. He's known her since he was a small child," said Santa Ana Police Corporal Anthony Bertagna.
The alleged acts happened in the 38-year-old's home in Santa Ana when the boy was only 15. Police suspect they were having sex twice every weekend for several months. Other people live in the house.
"The suspect lived there with her husband and three children, ages 7 to 17," said Bertagna.
Officers contacted by the alleged victim's mother this month. It comes after Tapia had given birth to a baby girl last November.
"The victim's mother had a DNA test performed and it turned out this child belonged to her son," said Bertagna.
Authorities say several months before, the victim's mother told Tapia to stop after she found letters from the woman in the room of her now 16-year-old son.
"She confronted her son and he admitted it had been going on for a while," said Bertagna.
Police arrested Tapia Tuesday morning.
Neighbors saw police but say they had no idea she was facing felony charges.
"Oh my god, it's scary," said neighbor Maria Rodriguez.
Tapia could not be reached for comment. Police say her husband bailed her out of jail Wednesday morning to take care of her baby girl.