Maria Gaetana Agnesi's younger sister, Maria Teresa Agnesi, was an accomplished musician and composer. I mentioned her briefly in this blog post. She was born on October 17, 1720 and died on January 19, 1795.
Maria Teresa Agnesi was one of the first Italian women to write a serious opera. Unfortunately, many of her compositions have been lost, but in recent years there's been a resurgence of interest in her and her music. You can now find clips of her music on YouTube, like this one:
I plan to add some details about her life, and how it influenced my novel, Playing by Heart, to this page soon. Meanwhile, you can read more about her here.