Short answers, pulled from the story.
Maria Anna Thekla Mozart, known as Bäsle or "little cousin," was the cousin and close friend of Wolfgang Amadeus Mozart. She was born in Augsburg on the 25th of September 1758, the daughter of Franz Alois Mozart, a younger brother of Leopold Mozart.
The Bäsle letters are ten surviving letters written by Wolfgang Amadeus Mozart to his cousin Maria Anna Thekla Mozart. They are notable for their abundance of scatological and sexual humor, and scholar Maynard Solomon translated a passage from the letter dated the 23rd of December 1778 into rhymed English.
Mozart and his cousin Maria Anna Thekla Mozart met in Augsburg between the 11th and the 26th of October 1777, when he was twenty-one and she was nineteen. They developed a close, probably intimate relationship during that time.
Yes. After Mozart's return from Paris, Maria Anna Thekla accompanied him in January 1779 from Munich to Salzburg and stayed there for several months. The visit occurred despite the disapproval of Mozart's father Leopold.
Maria Anna Thekla Mozart died in Bayreuth, Germany, on the 25th of January 1841 at the age of eighty-two. She was buried in the municipal cemetery in Bayreuth, though her grave can no longer be found. Two memorial plates were later placed in her honor, one at the Old Coaching House and one at the cemetery entrance.
Maria Anna Thekla Mozart remained unmarried her entire life. In 1784 she gave birth to an illegitimate daughter, Maria Josepha, whose father was canon Dr. Theodor Franz de Paula Maria Baron von Reibeld, who provided generously for mother and child.