What do we know about Shirou's biological ancestors? (not Kiritsugu) What do we know about Shirou's biological parents/grandparents/etc.? Do we even know anything at all? 

This is relevant to e.g. understanding why Shirou can perform magecraft despite presumably having had non-mage parents. (Also, it's just kind of weird that he seems to never say anything about them. He was certainly old enough to remember them when they died in the fire at the end of the fourth grail war.)
The identity of Shirou's biological family was never revealed in the game, the anime or the manga of Fate/Zero and Fate/Stay Night.
The only thing you know about them is that they died because of the end of the 4th Holy Grail War.

As for the reason why Shirou can perform magecraft, It is caused by the sheath of Excalibur : Avalon, which have been implanted in Shirou's body by Kiritsugu. Even though it is not clearly explained.