As you know, I, too, was surprised he didn't kill her when he had the chance. It did not seems a wise decision not to do so. Still, I'm not sure how readers would feel about a character who killed his own mother, crazy and dangerous though she is.
But the reviewer seemed to think that Kare could deal with the "bad guy" as easily as he could anyone else. What good would it have done to "break her arms" when it was her mind that was the danger.
And where did he get the idea that the premise of the books was that Averrine would be able to brainwash Kare into being another person entirely if she'd raised him from a child? Many people (ever those who don't have parents who can manipulate minds) grow up to accept their parents' values and espouse the same causes simply because they have only heard that one side of things since they were small children. Parents can't change who their children are, but they can influence what they believe. I never had the impression that Ealyn feared her influence on them would be more than that. Some children ultimately rebel, of course, or come to embrace different values once they get out into the world and are exposed to other ideas, but I don't see her allowing her children to be exposed to other ideas, and they were still young enough in Ealyn's vision that rebelling against authority, just because it was authority and stifling, might have been later on in that Kare's future (although one shudders to think how Averrine would have dealt with typical teenage rebellion).
But maybe I am the one who misinterpreted, and in the story she could have molded the children more than I think, if she had raised them. Was that what you meant to imply? Is that why Ealyn stole the children and was willing to sacrifice himself and Karia to keep Kare out of his mother's hands?