My DD (just turned 16) is Katelyn and the ONLY people who ever call her anything different are her brother who sometimes calls her Kate and my father who calls her Katy. To everyone else, she is Katelyn!
My son (22 years old) is Jared and that is what he's always been called. Well, that is until about 3-4 years ago when at work he started going by JC (first and last initials). It was difficult at first....we'd call him or stop by (it was a restaurant) and ask for Jared and nobody knew who we were talking about! Everyone in the family still calls him Jared though, not JC.
My sister is Martha Charlene. Growing up she always went by Charlene. For a while, probably late teens-early 20's, she went by Char. For the last probably 30 years she has gone by Charlee. My parents, sister, and I still call her Charlene, though!
My point is that it's just what you get used to. If your sister only calls her DD Constance, chances are that is what others will call her also. That is, until Constance decides for herself that she wants to be called something else!