Many things can affect a girl/woman's period asides possible pregnancy. She could not be eating enough (would have to be quite a dramatic change in eating though). Has she lost a lot of weight recently? That affects it too. Some dru8gs affect menstruation (both medicinal and recreational), and stress also plays a part. It could, also, just be a bit late. How old is your sister? If she's under 19 then it's completely normal for her period to be a bit crazy for no reason. If you guys are worried you could to the doctor.
I came across a list of ten things that could make a period come late I can't member them though.. I know if you take strong medication, lose weight fast, gain weight fast, have stress, that it could affect your period I can't member the rest.
A lot of things: Stress, a lot of exercise can also delay it, if they have eating disorders they won't get theirs, if she's preggo... or maybe their cycle just delayed a bit :P it varies from girl to girl :)
Be on the pill, you get two types: one that makes you regulate to have 1 week bleed and the second option cuts your period off completely
Depo shot and I know there's a couple differant birth control pills that will to, I don't know there names though
this question is my sister. I need to know what else can stop her period other than being prego.
A lot of exercise delays periods.Also sometimes birth control may delay or stop your period.
Emotional distress can also cause late periods