No they are not. They use some of the same characters, but they are not related in more then that. I believe all that is really from CCS in TC, is the characters and I believe Sakura is the princess of Clow Kingdom.......which has the word Clow in it. Now XXXHolic does crossover with TC at some points, in the manga anyways not sure about the anime on the XXXHolic front.
__________________  "April sweet is coming in,
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A Serious House on a Serious Earth
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