Lets take a look at another misunderstood story.  Gen 38.   Judah and his daughter in law, Tamar and the twins.

This is a chapter set in the the middle of the story of Joseph.  Joseph was sold to the Egyptians then suddenly comes the story of Judah and Tamar.   So some years must have gone by because Judah, Joseph's older brother was married and had three sons, two of marrying age.  The oldest son Er, was married to Tamar.  But Er was wicked in the LORDs site (notice all caps which means Yawh, the eternal God of justice).  For his wicked ways God killed Er.  Now lets think about that.  If God bumped off all the wicked people there would not be a lot of people around and those that were left would be smart enough to toe the line.  

Now in those days if a married man died and left no children then his next youngest brother was the kinsman redeemer and had to take in the widow and provide her with children.  In this case the kinsman redeemer was the younger brother Onan.  So we will say Onan was married to Betty.  It does not say this in the story but we can take some latitude here.  Onan and Betty had children and so and children Onan and Tamar would be considered Er's sons but Onan would be responsible for them and they would be Onan and Betty's heirs.  Onan did not want to have responsibility for children that would be considered his dead brother's children.  It was a grave sin to allow a married man to die without having produced children and his wife would be thought to be sinful and out of favor with God for being barren.  She could not go to worship and was considered then cut off from God and thus dead.  Whenever Onan slept with Tamar he spilled his seed on the ground.  So naturally God killed Onan.  Notice the swift justice here.  The third son was a little boy and his name was Shelah.   Judah had seen enough.  His sons were dropping like flies around Tamar so he sent her to her father's home to live as a widow until Shelah grew up.  Judah had no intention of risking another son.

Long story short, Tamar needed a baby to fulfill her obligation to provide Er with a son.  So she disguised herself as a prostitute and when Juduh saw her he had sex with her.  What is a guy to do.  You see a working girl and you have to help out.  Three months later Tamar was showing and so was thought to be a prostitute and so Judah said burn her to death.  Swift judgement again.  Tamar proved that judah was the father and he declared Tamar to be more righteous than myself because she found a way to have a baby that was part of the family thereby redeeming Er's honor to have son.  Got to be a son you know.

She had twin boys, Perez and Zerah.  You are way ahead of me because you know that Perez is in the linage that leads to Jesus.  

We also know that this must be a priestly source.  The punishment of Yawh is swift  and deadly.  We also know the sin that killed Onan was not spilling his seed on the ground as many puritans believe but failure to honor his dead brothers requirement to leave an heir as prescribed by the law.  And Tamar was blessed with twin boys for she was the most righteous of all.   If spilling one's seed on the ground was punishable by death there would not be a lot of people around and that would defeat the "be fruitful and multiply" rule.

Then the scriptures jump right back into the story of Joseph.  This is an example of how all the sources JEPD were dove tailed into one another.  Often the coalition meant verses were intermixed into one another making for confusing reading.