Is it bad that I havent got my period yet?

Hi! Im Gracie, my names Melinda but people call me gracie. Im 12 and i still dnt have my period my twin sister Mellissa got hers a couple of months ago and most of my friends hav theirs too! My dad says that all girls get it sooner or later, but hes a boy so im sure he doesnt kno much about stuff like this. My dads girlfriend katie says that its jus bcause im small for my age but Lissa's small too! I kinda dnt want my period bcuz when Lissa got hers she cried and said it hurt real bad. Im scared. But its still not fair that Lissa got hers before me. I dnt really want my period but I wish Lissa didnt get hers either she gets everything first. Why dont i have it yet? is something wrong with me?