Did Mozart have a sister?

Not that whether or not he had a sister was that noteworthy, but I had a band director in middle school who claimed that Mozart's sister was as much of a musical prodigy as the great Amadeus himself.

The only reason she was not as famous as her brother, this band director said, was because at the age of seven or eight, she was yanked from the world of music and began training in the wifely arts.

Any truth to this? What was this sister's name and does any of her music still exist?

Yes he did have a sister, whose full name escapes me at the moment but her nickname was Nannerl. (Mozart himself was called Wolferl when young, according to the EB.) Her descendants are Mozart's closest living relatives and some of them are pretty musical as well.
As for why she left the music scene early - no idea, though the theory you put forth is certainly plausible enough given the era. I've never heard of any of her compositions surviving, but misattribution is always possible. The "Toy Symphony" (with that annoying little cuckoo song that ends in a way presaging Spike Jones by two centuries) was originally thought to be written by Schubert (IIRC) but was proven to have been written by Leopold Mozart (Wolfgang's daddy).
So it's possible that something out there may be written by Nannerl but has been misattributed.

------------------
All Hail Unca Cecil, or the next best thing available!

At least we can be thankful Celine Dione pursued music instead of the wifely arts.

Cheese head I certainly hope that was sarcasm, Celene Dion is a wailing harpie!

------------------
To deal with men by force is as impractical as to deal with nature by persuasion.

Celene Dion a harpie? Come on! You must not have heard Alanis Morisset lately...

Her name was Maria Anna Walburga Ignatia, a.k.a. Marianne, and Nannerl to familiars. (There were also 5 deceased siblings). Marianne and Wolfy toured together until Marianne was 17. Then she drops from the touring scene. Wolfgang was still young enough to be a novelty, and contiued touring with his father. Being the girl also pushed her into the shadows, as the name, etc. followed the boy. And, although she was very talented, Wolfy was really the musical genius.