Complex sentence: His first wife was Sybilla Gronamann, a German singer; after her death he married the singer Charlotte Brent in 1766; she had been a pupil and mistress of Thomas Arne.
Split into simpler sentences: His first wife was Sybilla Gronamann, a German singer. After her death he married the singer Charlotte Brent in 1766; she had been a pupil and mistress of the composer Thomas Arne.