Statistically speaking, it is true step-parents are the most likely to abuse. Theories abound about them not having a biological link and in some cases even threatening existing/the creation of biological progeny. But abuse from male parents, step or bio, is almost always primarily/initially physical. The emotional and chemical abuse of this case is very much a mother (bio or step) abuse. Not to say it can't happen, just the likelihood is weighted in the one direction.

Now if the kid's face was punched, that'd imply step-father. Chemical castration? Not so much.