When Mozart moved to Vienna in 1781, and lodged for a time with the Weber family, he seems to have flirted with both Sophie and Constanze (whom he eventually courted and married). The incomplete Allegro in B-flat K. 400, written by Mozart at this time, contains (in W. Dean Sutcliffe]'s words) "a self-contained melodic episode in G minor, with the names of Sophie and Co[n]stanze Weber inscribed above a pair of prolonged sigh figures." In a letter of 15 December, 1781, Mozart described Sophie as "good-natured but feather-brained." In 1782, when Mozart and Constanze were married, she was the only Weber sister who was present at the ceremony.
In December 1791, when Mozart died, Sophie was 28 years old, and was the only daughter in the family still unmarried. She lived with Cäcilia, but was frequently present in the Mozart household during the composer's final decline and death, and helped Constanze care for her dying husband.