Back then, women had no protection unless they married--only sons inherited property.  And if their husband died, they were destitute unless they could remarry.   So, they had a rule among the Israelites that if a man died, the next brother in line would take the widow to be his wife.

Leah's son Judah married a woman named Shuah, and they had three sons.  Judah found a suitable woman--whose name was Tamar--for his oldest son El to marry.  But El was very evil in God's sight so God "slew" him.  As a result, Judah gave Tamar his second son Onan as a husband.  Who was also displeasing to God.  So he also died.  Leaving Tamar a widow for the second time.  As a result of this, Judah told Tamar that when his third son Shelah was a bit older, that she could marry him.  But when Shelah was of age, Judah reneged on his promise and left Tamar with no one to care for her.

So Tamar devised a plan to to put Judah to shame for leaving her with no husband.  "For she saw that Shelah was grown and she was not given unto him to wife."  She took off her widow's clothes, put on a wrap and a veil, and went to town and sat in an open place.   When Judah came by, he asked her what her price was--he didn't recognize her, and assumed that she was a harlot. (So much for Judah's morals.  What was he thinking!!  He should have been tending his livestock.)

She asked for his signet, his bracelets, and his staff.  Which he gave her, and also promised her a kid goat as well.  Three months later, someone told Judah that his daughter-in-law was pregnant and he called for her to be burned for her immorality.  "Who is the man that is the father of this child," he asked her.  She held up the signet, bracelets, and staff, and said, "Discern, I pray you, whose are these?"  And of course, Judah knew that he was the one who got her pregnant.  He was caught.

Judah had not given Tamar his son as a husband--as custom demanded, but he admitted what he had done and said, "Tamar has been more righteous than I; because I didn't give her my son to wed." (I have some question as to either of their morals--but back then, women who had no husband  turned to prostitution as their only means of support.  She is listed as an ancestor of Jesus.  Forgiven.)

Tamar gave Judah twin boys, but once again, the birthright would be in doubt. The baby that seemed to be coming first was marked with a red thread--but did not come, and was actually born second.