How is Clow Reed and Fei Wong Reed related in Tsubasa Reservoir Chronicle?
According to the Tsubasa Wikia:


  Fei Wong Reed also mentions Clow quite frequently, and he's also a descendant of Clow.


This is shown in chapter 64, which appears in Volume 9 of the manga. He is described as somewhat of a distant relative of Clow Reed.

Information for this can be found on: Fei Wang Reed's Wikia page and Fei Wang Reed's section on Wikipedia. The citing is provided on the Wikipedia page.