Nannerl! She was a prodigy nearly as accomplished as her brother Wolfgang, and as a young girl, she accompanied him all over Europe, playing for royalty and nobility. He was the greater musical intellect, no doubt about it, but she was highly talented as well.
So why don't we know more of her? For a number of reasons, the first being that she was female. As a young girl, her novelty as a prodigy was exploitable, but as she got older, society had different plans for her, and as soon as she became an adult, she retired from public life. She still taught lessons, and composed music, although none of it survives today.
There was also the matter of competition. There were literally thousands of musicians making a living in Vienna at the time, and that was just one city in Europe, which was filled with similar cities. So as talented as she was, she still had to compete with thousands of MEN, wherever she lived. No matter how talented she might be, she was never going to get a fair break. In fact, the more talented she was, the more her competition would have resented her, and fought her.
So she was destined to remain in her brother's shadow, a footnote in his life story.