Short answers, pulled from the story.
Sophie Weber (1763-1846) was an Austrian singer and the younger sister of Constanze Weber, who married Wolfgang Amadeus Mozart in 1782. Sophie was present at the Mozart wedding and was with Mozart during his final illness and death in December 1791.
Sophie Weber wrote a letter to Georg Nikolaus von Nissen, Constanze's second husband, to help with the biography of Mozart that Nissen and Constanze were preparing. The Grove Dictionary describes her remembrances as "moving." She was also interviewed in 1829 by Vincent and Mary Novello, who were gathering firsthand accounts of Mozart's life.
Yes. An incomplete Allegro in B flat, catalogued as KV 400, contains what scholar W. Dean Sutcliffe described as "a self-contained melodic episode in G minor, with the names of Sophie and Costanze Weber inscribed above a pair of prolonged sigh figures." Mozart composed it while lodging with the Weber family in Vienna around 1781.
Sophie Weber married Jakob Haibel (1762-1826) on the 7th of January 1807 in Djakovar, Slavonia, the town now known as Đakovo in Croatia. Haibel was a tenor singer, actor, composer, and cathedral choirmaster who had written a successful Singspiel performed by Emanuel Schikaneder's theatrical troupe.
Sophie Weber died in Salzburg in 1846 at the age of 83. She had moved there after her husband Jakob Haibel's death in 1826 to live near her sister Constanze, and she outlived all her sisters as well as Mozart's son Franz Xaver Wolfgang Mozart.
In a letter dated the 15th of December 1781, Mozart described Sophie Weber as "good-natured but feather-brained." At the time, Mozart was lodging with the Weber family in Vienna and reportedly flirted with both Sophie and her sister Constanze.