What do we know about Shirou's biological ancestors? (not Kiritsugu)
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.