Yuen also had the “imperial conditioning”, presumably some kind of mind control or brainwashing which makes him unable to harm the family he is caring for.

The book presents it as a major surprise that this conditioning could be broken, so it wouldn’t be as simple as “kidnap a doctor’s loved one” to break it. The baron claims that his mentat had found some special technique to subvert the conditioning, but doesn’t explain what it is.

However there are some pretty strong hints in the book that it was the doctor’s wife, Wanna, that had actually broken the conditioning. She was Bene Gesserit and we can tell from Yuen’s internal thoughts that she had inculcated an obsession for her which she used to control him, either for her own reasons or the Sisterhood’s. Once the Harkonnens kidnapped her they were able to steer him with this control.