Clamp appears to have deliberately chosen not to give us any details regarding the family. I don't suppose they thought it necessary for us to know as it distracts us from the storyline and gives fans things to chew over after watching the show, that's probably partially why CCS has a very large number of fanfics present. Anyways, I find Mozey-chan's theory most interesting:-
My theory is that she was forced into an arranged marriage, considering they were born into a very well to do family, and she was really in love with Nadeshiko. Eventually, I'm assuming, her parents died, as they are never mentioned in the anime or the manga, and she divorced her husband because she's actually a lesbian.
It's a pretty well known fact that homosexuality is hereditary and obviously Tomoyo also inherited it, and very well likely that Touya inherited it as well since Tomoyo and Sonomi are his and Sakura's cousins.
But if Sonomi is a lesbian, then how did she have Tomoyo in the first place?? Of course, there will be various alternative methods that she can use, but the possibility of an adopted daughter, though striking me as pretty far-fetched, appears scarily possible.
And another thing is Fujitaka. Like him, Eriol is the reincarnation of Clow, yet does not appear to have any immediate living family, giving me the impression that he just simply popped out from somewhere. Unless Clow Reed actually designated a person to give birth to his reincarnaton (which is possible I'd suppose), I wonder if Fujitaka is the same, having appeared out of the blue, his destiny to find the Clow book and bear the girl that would take over the Cards.
And last but not least is the issue of the Tachibana family. My memory on this is hazy so please correct me if I'm wrong. Anyway, we are told during Sakura's encounter with The Silent of Yuki Tachibana and her artist dad Tenkai Tachibana. And later in the series when The Dash appeared, A girl called Rei Tachibana takes care of the injured Dash! Is perhaps Tachibana just simply a common Japanese surname???
