What if your mother-in-law turned out to be an evil, cold-blooded witch . . . literally?
Hmm, that's kind of a tall order, given how many layers of figurativeness are stacked on the term "cold-blooded." Is the witch literally endothermic? Does she have too little energy to function in winter weather or at night? Then again, endotherms are not literally cold-blooded except in a cold environment. So for that description to be entirely literal, her blood would have to actually be substantially below room temperature at all times, which raises all sorts of issues of its own.
Maybe the description should've avoided the many possible layers of "literally" abuse by just saying "...turned out to be an evil witch, literally."