Well lets see, I'm a Fuffy shipper so I'll just ignore the sister thing. But I do think that they had a type of bond that is so much different from every single other character. And I'm not saying this while talking about the sexual tension, I'm refereing the the complexity of the relation overall. They were indead the ying and the yang, they almost represented two face of the same coin. Even when they were at the worst in their relation ship, like right after buffy stabed Faith, there was still a sense of understanding and even a weird sense of love for each other(not necessarely romanticly) Even though they almost became wrost enemies at one point, remember that faith had a couple of chances to hurt or kill Buffy but she didn't for the simple fact that its probably the one thing that Faith couldn't accept doing. We also saw in the dreams they kinda share that they were connected and by killing each other they would automaticly destroy their own life. If Buffy had in fact killed Faith, I don't beleive that she would have forget herself for it even if she didn't seem to care at the time and if Faith had killed B, there was no way she was coming back from the darker side of herself.
They also both always found a way to be at peace with each other in some way. the kiss that Buffy gave to comatized Faith was a bit like her forgaving her and understanding Faith's decision. It was like a closure (even though Faith woke up later) and this kind of peace came back eventually after each event that lead them appart. Like in season 4, she then went to jail after. And when she came back in season 7, they reconcile (even if the relation ship would never be the same, at least they weren't at war anymore).
Anyway, I could go on forever for the simple fact that there relation ship is probably even more complicated then a sister kind of bond.
-Blaze-