Blood pressure is an important sign that we as doctors follow. It is a "vital sign" as it is important regardless of if the patient is pregnant. If there is any concern I would STRONGLY recommend your sister see her OB/GYN.
Both high and low blood pressure can be a problem. Blood pressure is the ultimate end product of the cardiovascular system (heart + blood vessels). The blood pressure is important because it is the driving force that allows for the blood to bathe and supply the organs of the body. If it is too high--it can damage the organs. This is specifically important in pregnancy as it can damage the placenta--the food source of the baby. This can result in growth retardation. The flipside, or low blood pressure is also very concerning. This not enough blood is getting to the organs. The fact that she gets faint can suggest she is not getting appropriate blood to her brain. If there is not enough blood to the placenta again problems can occur.
Keep in mind blood changes during pregnancy because of all the hormones. Regardless of the cause if she is having symptoms--this is a problem. Also note, high blood pressure
in the later parts of pregnancy can be consistent with preeclampsia--a serious condition.
I strongly encourage you to talk to your doctor