Digi: Its best to forget
I don't know, my mom and my uncle had an age difference of 19 years. My oldest cousin is a good 20 years older than me. Its totally possible for Peggy's parents to have had her sister later in life and hence Sharon be mid 20's.

Also when do they ever say how old Peggy is? I always assumed Peggy was way older than Steve if the movie for no other reason than her rank.

And on a totally different note, it could have been even more annoying if they went with the original backstory for Sharon and made her Peggy's younger sister.