history of Joseph is in the Bible because it IS a family history.
For see, it is the history of a man who loved his family, who felt
that family life was holy and God-appointed; whom God rewarded with
honour and wealth, because he honoured family ties; because he
refused his master's wife; because he rewarded his brothers good for
evil; because he was not ashamed of his father, but succoured him in
his old age.

It is the history of a man who--more than four hundred years before
God gave the ten commandments on Sinai, saying,

Honour thy father and mother,

Thou shalt not commit adultery,

Thou shalt not kill in revenge,

Thou shalt not covet aught of thy neighbours--It is the history, I
say, of a man who had those laws of God written in his heart by the
Holy Spirit of God; and felt that to break them was to sin against
God. It is the history of a man who, sorely tempted and unjustly
persecuted, kept himself pure and true; who, while all around him,
beginning with his own brothers, were trampling under foot the laws
of family, felt that the laws were still there round him, girding
him in with everlasting bands, and saying to him, Thou shalt and
Thou shalt not; that he was not sent into the world to do just what
was pleasant for the moment, to indulge his own passions or his own
revenge; but that if he was indeed a man, he must prove himself a
man, by obeying Almighty God. It is the history of a man who kept
his heart pure and tender, and who thereby gained strange and deep