I am also a big fan of Mozart and did not have to look her name up... but then again... as I am a particularly nerdy human being, I tend to enjoy biographies of famous people I admire. I referred to her as "Mozart's sister" because I wanted people to know who I was talking about. It was supposed to be kind of funny though.
I do agree with Claire, however, because she was, in all likelihood, not allowed as much access to a piano, she was forced to stop playing to be married... and the second her brother showed any sort of exceptional apptitude, he was the only one their father booked on tours. In the beginning, she was just as famous as he was... their father just chose to use her as "side dish" to Wolfgang's compositions. If she had lived 250 years later, I think it is very possible that she could have been considered just as much of a genius as her brother.
I very very much disagree with your notion of the women I named being "tinkerers", Virulent. With the exception of the woman who is told to have invented Kevlar (honestly of COURSE the man says she wasn't helpful. why would he want to share credit, especially since she is the one usually credited with the invention anyway), by your own definition of genius as being given for services rendered, why are these inventions less important than any other? Honestly, the inventions I named -antibiotics, petroleum refining methods, pollution reducing items, Madam Curie's findings, human stem cell researchers? That's not important? I truly do not understand why those are less important contributions than Mozart's music.