Your Mother-in-law is 46yrs and also in menopausal age gr. In this time, continuous bleeding for 15 days may indicate some underlying pathology.
As your scan report does not show any uterine pathology,then you need to rule out cervical or vaginal pathology.
So do PAP smear
test along with DC biopsy
with consult your gynecologist.
All the best.
For more suggestion,mail me...firstname.lastname@example.org