August told her he came through the wardrobe with her. She didn’t believe at the time but once she believed in the curse I’m sure she would believe that as well.
Emma doesn’t know about the “one person” rule of the wardrobe (at least I don’t think so. She’s skimmed the book but I don’t think she full on read it). So perhaps in Emma’s brain she’s thinking her parents entrusted her fate to a 7 year old boy and then he abandoned her too…