The following is from the Wikipedia article about Aloysia Weber:
Maria Aloysia Louise Antonia Weber (c. 1760 – 8 June 1839) was a German soprano, remembered primarily for her association with the composer Wolfgang Amadeus Mozart.
Aloysia grew up in Mannheim and later moved to Munich in 1778, where she made her operatic debut. Her salary at the Court Theater was 1000 florins per year; her father made 600. The following year she was engaged to sing in the National Singspiel in Vienna, a project of the Emperor Joseph II; the family moved together to Vienna in September, where the father worked briefly as a ticket-taker, but died suddenly only a month after their arrival. Aloysia continued in a fairly successful singing career in Vienna over the next two decades.
On October 31, 1780, she married Joseph Lange, an actor at the Court Theater who was also an amateur painter (he later produced a well-known portrait of Mozart). Since she was the main support of her family at the time, Lange agreed to pay her mother the sum of 700 florins per year on a continuing basis, as part of her marriage contract.
She spent her old age in Salzburg, in order to be near her surviving sisters Constanze and Sophie, who had moved there.
She was for some time a love interest of Wolfgang Amadeus Mozart. This was around 1777, when Mozart spent some time in Mannheim, where he had hoped (in vain, it turned out) to find employment. Mozart expressed a desire to marry Aloysia, though it is not clear exactly how serious his intentions were, or whether they were reciprocated.
Mozart left Mannheim for several months for Paris on an unsuccessful job search. On his way back to Salzburg, he passed through Munich, where Aloysia was by now employed. According to the tale told in Georg Nikolaus von Nissen's draft biography of Mozart written in collaboration with Constanze, who married Nissen after Mozart's death, Mozart and Aloysia had a rather unpleasant encounter:
The vulgar phrase in Mozart's song corresponds to the English idiom "kiss my ass", and occurs frequently in Mozart's letters.
Mozart himself moved to Vienna in 1781, and later that year was for a time a lodger in the Weber home. The father Fridolin had died in 1779, Aloysia had not left home at the time of her marriage, and the mother Cäcilia and the remaining three sisters were taking in boarders to make ends meet. Mozart fell in love with the third daughter, Constanze. When the two married in 1782, Mozart became Aloysia's brother-in-law. Apparently there were no long-term hard feelings, as Mozart wrote a fair amount of music for Aloysia to sing.
The following is an insert aria composed for Aloysia by Mozart entitled Vorrei spiegarvi, oh Dio!. It's easy to tell from the music what a kind of a voice Aloysia had and why she was such a popular singer. Mozart most certainly was fond of her voice, for he composed several of his most beautiful works for her.