PERIOD IS M.I.A

I'M TRYING NOT TO FREAK OUT, I haven't gotten a period since March. I've always been extremely irregular but I want to say this has been the longest without menstruating and it's freaking out to the point that I think something is wrong with me as a lady. I'm 20, and to give background my little sister was diagnosed with Leukemia back in March, I've read stress levels tend to affect our cycles , my mom hasn't menstruated since April, Could this be affecting us both? what else could it be (fertility issues) and what can I do to someone trigger it or summon it lol :( (and no not pregnant I've checked like 5 times weeks apart also use rubbers sooo)