Very odd

So I need sum advice on a situation. My sister has been telling me how for about 2 yrs she has not gotten her period & that the "doctors" said she is good there's zero bad going on inside & she's not pregnant.... I wanna know how in the heck that can be??? Us having a period is cleaning u out of all the yuck inside our body that we don't need how is it possible for a 22yr old to not have a period??? I was thinking maybe her weight but I am a big girl too & I still get mine all the time I was only late 2 times in my 26yrs....I tried to research but zero...Help plz!!! My last resort might be to take her to a doctor myself cuz I just can't see a doctor telling her her insides r fine....