By Alex Prewitt At least 76 pairs of siblings have participated in Beijing, turning the Winter Games into a family affair with nearly 5% of the total athlete headcount. BEIJING — Christian and Scott Gow have been together since Day One, if not in birth then at least in biathlon. Now 31 and 28 years old, respectively, the Canadian brothers first signed up for the sport at the same summer camp. They have trained together at the same facilities, raced together at the same events, reached the World Cup circuit at the same time and, most recently, finished sixth in the men’s 4×7.5-kilometer relay a…