My sister is 20 weeks pregnant she nearly fainted at work as a teacher she was sweaty pale and felt sick and vision faded is this normal and common?

Probably: Sometimes pregnant women are sensitive to things like standing or sitting too long, hot showers, skipping meals or not drinking enough water. All of these things can cause a drop in blood pressure that causes the symptoms you described for her. If your sister doesn't think that any of these apply to her, she should let her doctor know.