A friendly sibling rivalry turned out favorably for Mark Stuart and the Boston Bruins. Stuart’s goal broke a tie a period after brother Colin scored his first NHL goal for Atlanta, and the Bruins ended a six-game skid with a 5-2 victory over the Thrashers on Monday. “It was nice,” Mark Stuart said. “He one-upped me earlier so I had to do it.” Boston’s first goal of the game — a power-play score by…