My sister is 3 months pregnant but she still has a period is this normal and what causes her to still bleed?

Possible problem. : Bleeding in early pregnancy sometimes is sometimes caused by the embryo implanting on the wall of the uterus. Sometimes can be a signal of a possible miscarriage. Sometimes means there is a small blood clot in the uterus that is slowly coming out. An ultrasound would tell what is going on.

Not normal: It is not normal to have periods when one is pregnant. The bleeding she is experiencing is probably not her periods.