Short answer: No.
Technical answer: If you really want to, you can use it.
The word is the winner of a contest held by the publishing house "Duden Verlag" and the soft drink manufacturer Lipton in 1999. The aforementioned (German) Wikipedia page tells us that it was included in "the Duden" (a popular orthographic dictionary of the German language) for a while, but has since been taken out after it didn't enter broad use.
An interesting side angle on this is that until 1996, the Duden was normative (for Germany, mind you), i. e. being in it would have made the difference for a word being a German word or not. In 1996, there was a reform of German ortography (English Wikipedia page), which, besides introducing new rules, removed the normative roel of the Duden.
Moreover, the reform sparked a controversy ending in a court ruling (!) that stated that orthography is not a law and therefore everybody can use the language however they want. (This probably seems very funny to native English speakers, who don't even have a normative body for their language.) In particular, that meant that big newspapers (who were important drivers of the controversy) are not required to follow the new rules (I can hear the folks from The New Yorker giggle quietly*).
So technically, when
sitt was "introduced", there was no binding normative body defining the German language anymore. It was not added to the definition of German taught in schools. Since it's not seriously used by anybody, we can well consider it a marketing gag rather than a German word.
*see this hilarious article on why