Nickname trouble?

I may just be being nit-picky or hormonal (I'm 8 months pregnant)... my toddler's name is Sienna. My husband's side of the family are huge on nicknames and we all call her Si-Si. I think it's cute, but JUST a nickname. I primarily call her Sienna and my husband/his fam call her Si-si. On every single holiday card, letter, or keepsake from his family... they write "Dear CeCe" or "CeCee", which just annoys me because it's not her real name or even close to the spelling, and makes me want to throw away the card. Am I just an overly-bored and hormonal pregnant girl or would this irritate you?