I vote for Sharon Carter. I don't think it has to be creepy, and here's how.
Don't make her a love interest right away.
Have her as someone working for the big bad. Cap is intrigued by her due to her resemblance to Peggy so he gets swept up in the movie's main plot. But his interest is never romantic.
Maybe have Sharon attracted to him, but nothing comes of it. They end the movie with her, Steve and Sam (gotta have Sam) as bros.
Then in the third movie, have them fall in love toward the end.
And I think Hayley Atwell should play her. Her resemblance to Peggy is pretty important, it is her resemblance to Peggy that makes him notice her in her first appearance as she's walking down the street, undercover. He only learns they're related much, much later in the comics. and to have some other actress, be she Cornish, Righetti or whoever referred to as resembling Peggy makes it look creepier, not less.
Otherwise just nix the familial relationship altogether. Carter is a pretty common name, so it could just be an odd, awkward and uncomfortable coincidence, or alternatively, call her Sharon Ferguson or give her some other last name.
Also, Hayley Atwell passes the jean and tee shirt test, so there's that.