Judah and Tamar

Genesis Chapter 38

At this stage the Bible takes a sideways step and Joseph is being a slave in Egypt we get some background on his half brother, Judah

The Sin on "Onanism" (and a song by Monty Python’s Flying Circus) are based on this chapter. 

Judah has set off on his own and married a local girl and has 3 sons, Er, Onan and Shelah. 

Judah arranges a marriage between Er and a woman named Tamar, but “Er was wicked in the Lords sight, so the Lord put him to death” Not sure what for exactly.

Jacob then says to Onan that he must do his duty and marry his sister in law so the line of Er would not die out. 

However Onan decided that any child of this union would inherit the family estate as he would be counted as Er’s child rather than his so he spilled his seed on the ground. This was a wicked thing to do as well and God puts him to death as well.

From my reading, Onan's evil was greed and not being willing to carry on his brother's name (which was a big deal back then) rather than spilling his seed on the ground.

Judah then tells Tamar that she should go back and live with her parents as a widow until Shelah is grown up. 

Tamar does as suggested but eventually figures out that Judah has forgotten her, so she dresses up as a prostitute and old father in law decides he feels the urge, and not recognising his daughter in law, gives her his staff and seal while he goes back and gets some money to pay for her services.

Tamar gets pregnant and when Judah finds out he, quite hypocritically, goes ballistic and demands to know who the father is before she is stoned to death.

Bringing out his staff and seal, Tamar actually embarrasses Judah into not killing her

Tamar is the first of 4 women who are Jesus direct ancestors that are listed in the gospels.