Originally Posted by shza

TSA Precheck does not come automatically with MCS. It's separate, and determined by TSA. If she has Global Entry (highly recommended) or anything else that gives her a Known Traveler Number, then that's why she's precheck. Otherwise, it's "random" (I have heard that airline status and premier seating are sometimes taken into account -- though it seems beyond silly to assume that a hijacker wouldn't buy an economy plus seat).

Just FYI, premier line check-in/security *does* come with MCS, so your sister should actually have both on her boarding pass (mine always do -- though I'd never take the premier security line over the precheck line).