Hello dear,I understand your concern.
In my opinion the physical presence or absence of your sisters will not affect the periods.
But there are conditions which can affect your periods and cause delay in a regularly menstruating woman by causing
hormonal imbalance like :-
1)Physical or mental
stress.
2)Any infection.
etc.
If you are sexually active then the
pregnancy is ruled out by doing a urine pregnancy test.
As per your history you are not sexually active so there is no chance for pregnancy.
Usually the duration of menstrual cycle is 21-35 days.So check for the number of days and wait for spontaneous onset of periods.
Avoid stress if any and take healthy nutritional diet.
Check for your periods for 2-3 months.In case if irregularity continues then other causes like
thyroid abnormalities,ovarian abnormalities like polycystic ovarian disease(PCOD),
overweight or obesity,stress etc.
Nothing to worry as such.Relax.
Best regards....Dr.Srilatha