As we know, Sharon Carter, AKA Agent 13, will be making an appearance in Captain America: Civil War. She first came on the MCU scene during Winter Soldier as Cap's undercover flirty neighbor nurse. Sharon had joined SHIELD's Special Services, apparently reserved for Fury's top undercover agents who get only a number designation. Fury made sure that Sharon was placed in a position to watch out for Cap--probably aware of the family connection.
You see, we also know that she's supposed to be the niece of SHIELD founder/WWII bad-ass Peggy Carter. This is based on all the information that Marvel has given us so far, in conjunction with the original comics.
However, one thing I thought of is this: How can Sharon be the niece of Peggy if Peggy doesn't have any siblings?
She has the same last name as Peggy according to IMDB.com. Obviously, she has to be her great niece, as the age difference (in MCU terms) would be insurmountable for a direct familial connection -- Peggy's from the 1940's and young Sharon is from the 2010's. Rumors from months ago has Sharon at Peggy's funeral along with Steve Rogers and Falcon.
Well, this season of Agent Carter certainly gave us a possibility and maybe Civil War will fill in the blanks.
The only answer to this mystery is that Sharon Carter is the granddaughter of Michael Carter, the supposedly deceased brother of Peggy Carter. His "death" was the impetus that Peggy needed to join the SOE, then the SSR and SHIELD. I talked about my theory that Michael is still alive in my last post about Agent Carter. There was nothing in Agent Carter about Michael having a family before he was killed, so maybe he had his family during the time (or maybe after, depending if Season 3 goes ahead) he was supposedly dead.
Is this something that might be addressed in Civil War? Won't Steve ask how Peggy has a niece but her brother is dead (something she probably told him during their time together)? Well, Sharon's side of the family must have gone legit at some point, otherwise she wouldn't have been able to join SHIELD or the CIA...