He sexually molested her children. In the book, it doesn't say much more than what Lexie says in the movie, other than, "Roger was bent over the end of the bed. He was inside Praline, Novalee. Inside my baby," and, "When he tried to put it in Praline's mouth, she threw up on him, and that's when Brownie came in, so he took Brownie instead," so the book gives a clear definition of what is going on.