The following spoiler is from EW Spoiler Room:

Any word on when Kara will find out about Sam being Reign on Supergirl— Margaret

Considering Sam turns to Alex with her concerns over losing big chunks of time in an upcoming episode, I feel like we’re closer than ever to that reveal. “It does seem like it’s only a matter of time,” Melissa Benoist says. “What I think is the most important is how Sam is going to react, because obviously she has no awareness of what’s happening right now. The entire arc of the season is this identity crisis for every character, so if Kara finds out about Sam, if and when, it’s definitely going to be a part of that journey.” The question remains whether Kara will actually be there to support Sam through this twist. “That would be difficult,” Benoist says, “but knowing Supergirl, and how she tends to deal with situations, and how she thinks everyone deserves a second chance, and that you can change and be a better person and a hero, I think she’ll try to help her.”