Erbarm dich mein, o Herre Gott

Johann Sebastian Bach's chorale prelude on 'Erbarm dich mein' is probably one of his most famous organ pieces, you can find many recordings on many platforms of this work. However, this is a setting by J.S. Bach's great-uncle Heinrich Bach (the brother of JSB's grandfather). Heinrich Bach also was the grandfather of Bach's first wife Maria Barbara Bach.
Heinrich Bach is also called the father of the 'Arnstadt Line' when it comes to the family Bach. Two of his sons are also quite well known: Johann Michael Bach and Johann Christoph Bach.