My sister is 4month pregnant, she pass out suddenly very often, her ketone, sugar and blood pressure is normal, what's the cause and what to do?

No driving!: That is sure scary! If her ob has checked and no idea, try her primary care again. Be sure she is not driving, and be with her as much as you can. Remind her to use handrails, stay away from heights, keep drinking fluids, no driving. If she does it again and no answer found, to ER for another opinion and to be sure. No drugs or alcohol either of course.

Not normal: It sounds like she needs more of an evaluation to figure out why this is occurring. You can pass out due to a number of reasons, like blood pressure changing depending on position, abnormal pumping of the heart, abnormal heart rhythms, poor blood return to the heart, neurologic problems and other things too. Please make sure that this isn't ignored for her. Best of luck.

Fainting: See the doctor. It is not normal for one to faint during pregnancy. Possible reasons could be low blood sugar, low blood pressure, which you state have been determined to be normal. An electrocardiogram should be performed to rule out a cardiac cause. Perhaps there is something emotional going on? Pregnancy hormones can cause women to be very emotional, although fainting is not common.