The answer is simple.
Most other tennis siblings, had rather disparate skills. Chris and her sister, John and Patrick McEnroe. When one is near the top and the other is 30th or 40th, there is not much drama.
Venus and Serena, however, are both top ten players for the last four years, and in the last year or so threaten to dominate the larger high purse tourneys. They were the money leaders in 2001, in tourney winnings, despite Serena hovering around #9 almost all year. She won or placed well the right tourneys to take home the cash. Venus did this also.
Their ethenticity is a factor also, don't kid yourself.