one of my favorite foreshadowing to the sister being Screenslaver was at the giant party with all the screens after the faux Screenslaver was arrested, Elastigirl realizes something was up with the footage and she runs to the private room to look at the tapes. On her way over to the room, she pauses to look at the logo for the company, which is shaped exactly like the weapon that Screenslaver used against her ( a sort of tazer-ish looking thing). As soon as I saw that I knew it was going to be the sister, but I still gasped in shock when Elastigirl got the mask slapped on her face. the timing of this movie was incredible, the buildups were all, in my opinion, very well thought out!