Lawrence County police said the boy was charged with criminal homicide and criminal homicide of an unborn child, the New York Daily News reported Saturday.
Kenzie Houk, 26, who was 8 months pregnant, was shot with a shotgun in her bed about 8 a.m. Friday in the family's farmhouse in Wampum, the newspaper said.
"It's tragic," a relative who preferred not to be named told the Daily News. "They were to be married. He was jealous."
After allegedly shooting Houk, the boy rode the bus to school with Houk's 8-year-old daughter. The victim's body was found later that morning by her 4-year-old daughter, who ran outside and flagged down tree trimmers working nearby, the Daily News said.
The boy's father, identified as Chris Brown, was at work at the time.
"I wouldn't know anybody in the whole world who would want to kill a 26-year-old beautiful daughter like mine and take a child," Houk's father, Jack, told MSNBC. "I don't know who would be crazy enough and nutty enough to want to do something like that -- has got to be sick. I hope they find out who it is and they give them life or whatever the got to do with them, it's horrifying."