I'm 13 and i have a question about my period?

Ok so, i have been waiting for my period to happen for a while. I am in the 8th grade right now, and i feel like it will never come! All of the women in my family got there periods in 6th or seventh grade, and my younger sister who is in 5th grade already got hers! during 7th grade, i started to get cramps often, and had crazy mood swings, but i never get it then either. I have gone through everything for "puberty" (hate that word) except this part. Please help me other experienced women, what should i do? I know it's all about nature and stuff, and my time will come when it comes, but what should i do?