My younger sister has really big and mine are fairly small. WHY IS THAT!?

So my sister is 14, and she has D's! And here I am, 21, and I'm only a B. Why is this!!??!! She's a 36D and im only a 34B. Seriously, is this normal? My mom has big booobs to and is it weird i dont? also, is it normal for her melons to be so big? seriously, like we'll be walking down the street and guys whistle at HER not ME. WTF?