JKR: I see him [Dumbledore] primarily as someone who would be self-taught. However, he, in his time, had access to superb teachers at Hogwarts, so he was educated in the same way that everyone else is educated. Dumbledore's family would be a profitable line of inquiry, more profitable than sweet wrappers.
The only relative of Dumbledore's that has been mentioned in the books thus far has been his brother, Aberforth. We know he is the bartender at the Hog's Head Inn, he was accused of using inappropriate charms on a goat, he was in the orignal Order of the Phoenix, and he was there the night that Dumbledore heard the prophecy from Trelawney. Dumbledore has implied that his brother was something of an informant on the goingson of the wizarding world, he was seen with Mundungus while Mundungus waas caught by Harry stealing things from the Black family home, and he was at Dumbledore's funeral. Also, of some possible importance, is that, although we know of the familial connection between Aberforth and Albus, Harry does not. I have a feeling this may be of some importance to Book Seven.
Is Aberforth the 'profitable line of inquiry' we are supposed to make? How will Harry feel about finding out that Dumbledore has a brother? And what kind of relationship will form there?
And if Dumbledore's family is going to be so important, why have we heard nothing about any of them, other than Aberforth? Without the quote, we could just assume that none of his family was important, but now that we know this is not the case, what are we supposeed to assume? JKR has said that she will not introduce any new chracters in Book Seven, other than a member of the Order that we have not properly met. Could this be Aberforth?