Hello, thanks for using Health Care Magic.
The upper limit of menstrual cycle is 35 days and that of your sister is way above implying its no longer considered normal. The most common cause of absence of menses in sexually active women is pregnancy
whose first symptom is usually an absence of menses and confirmed by a positive pregnancy test or an obstetrical ultrasound
Concerning, your sister, if she has not mistaken her LMP and that she is actually 43 days past her LMP, a negative pregnancy will strongly be true making it less likely to assume she is pregnant. Ovarian disorders such as ovarian dysfuntion, PCOS
, ovarian failure, anovulatory cycles, and disorders in the brain-ovarian hormonal
axis are common causes of absence of menses.
What your sister should do is to stay calm, and re do the pregnancy test in two weeks. If it comes out negative then she will need to be investigated for other possible causes by a gynecologist.
Best regards and hope this helps.