It's unfortunately a very well known phenomenon.
This study is from the States:
The Fourth National Incidence Study of Child Abuse and Neglect
"According to the report, children living with their mother and her boyfriend are about 11 times more likely to be sexually, physically, or emotionally abused than children living with their married biological parents."
A 2005 Missouri study, published by the American Academy of Pediatrics, reports that "children living with a stepparent or unrelated adult were 50 times more likely to die of inflicted injuries than children living with both of their parents".
[broken link removed], found that the incidence of abuse
was an astounding 33 times higher in homes where the mother was cohabiting with an unrelated boyfriend than in stable nuclear families.
Leslie Margolin of the University of Iowa in the journal Child Abuse and Neglect. Prof. Margolin found that boyfriends were 27 times more likely than natural parents to abuse a child.
Here's some info on the
Cinderella Effect.