This is quite natural and she need not to worry about it.
The reason of bleeding after intercourse, in the early days of marriage, is rupture of hymen and reason for repeated bleeding is that the vagina gets swollen and inflammed after forceful stroking and repeated attempts of intercourse in a short time, which further aggrevates the swelling and pain. it causes bleeding also.
All ur sister has to do, is to consult a lady doctor for examination to rule out any genital disease plus she have to take vitamins, calcium and antibiotics given by the doctor. ask her not to apply anything in her vagina without dr's advise and donot use any drug as self-medication.