Answer: Frau Weber (his mother-in-law)
I don't know if this scene has any historical basis, but it is a rich moment in the film! Originally, Frau Weber was Mozart's landlady. He fell in love with her older daughter who did not return his affections. Then he turned his attention to Constanze, the younger. Maria Magdalena Hoffdeml is the focus of great rumor and speculation concerning Mozart. Although she is never seen or mentioned in the movie, Maria was Mozart's piano student and alleged mistress. One of the Mozart death theories has Maria's angry husband attempting to murder her, poisoning Mozart, and committing suicide. Maria was pregnant during all this, supposedly with Mozart's child.