My take on this question is this: When I watch a match I want to know that it matters a lot to both players whether or not they win. A lot. I love watching majors in which one of the sisters is playing. You can really sense how much she wants it at all costs. However, when they play each other, there's just no escaping the fact that they know and we know that the worst thing that can happen to either one is that her sister wins a Grand Slam. Seeing your sister, especially one who is also your best friend, win a major is a nice consolation prize. Oh sure, on some level, once the two are out there on the court, they both want to win, but deep down they know that if they don't, it's not such a big deal, since the second best result is a pretty good one, too.
That's a bit of a dilemma for the sisters, albeit a nice problem to have. For those who aren't diehard fans of the family, though, we'll probably just tune in when only one of them plays since we know it matters more to them on a personal level. The problem for us is that as long as they are ranked 1 & 2, we'll probably see these finals in majors for some time to come since they are clearly a level above the other players. They deserve to make it to the finals, and I'll enjoy watching them in their matches leading up to the finals.