"Jacob lay with Rachel also, and he loved Rachel more than Leah... When the LORD saw that Leah was not loved, he opened her womb, but Rachel was barren."
Just as Israel cannot serve two masters, he cannot serve two wives. The tension between sisters Rachel and Leah is sibling rivalry to the extreme as they both compete for the love and attention of Jacob.
But even though the situation is complicated, the LORD provides for Leah when Jacob ignores her. She bears him 4 sons in a row (Reuben, Simeon, Levi, and Judah) and Rachel is jealous. Rachel gives Jacob another wife, her servant Bilhah, and gains two sons (Dan and Naphtali) in response to Leah's four. Leah in turn retaliates with her own maidservant Zilpah, given her as a wife and gaining two more sons (Gad and Asher) and then has two more sons herself (Issachar and Zebulun) and a daughter (Dinah) after hiring Jacob's "services" with some mandrakes. The LORD finally opens Rachel's womb and she bears Joseph in her old age and dies while giving birth to Benjamin. Final score: Leah 8, Rachel 4, Jacob 4 (wives).
And yet, out of this brokenness came the 12 tribes of Israel (aka Jacob). Even in the complications, God moderates the situation to be fair to both Leah and Rachel. Jacob only wanted Rachel, one wife, but he received two. And take it from him, one is more than enough for anyone.