It is for good reason that co-wives are described as tzarot , literally problems. Competing, as they inevitably must for the same man, their relationship is doomed to one of jealousy, bickering and even unfortunately hatred. Jewish law, recognizing this sad situation disqualifies the testimony of one spouse regarding the other - we are simply afraid that they will lie.
Despite the best of intentions to make this "love triangle work" the Bible makes it quite clear that co-wives are doomed to failure. It was Sarah's idea that Abraham marry Hagar - she wanted here husband to have a child even if with another woman. Yet when Abraham actually married her, Sarah had her thrown out of the house. This backsliding by Sarah led, the Ramban (16:6) claims, to the ongoing persecution of Hagars descendants towards the descendants of Sarah.
Rachel knowing her father Lavan was about to try and trick Jacob, passed on secret codes to her sister, codes Yaakov and Rachel had developed to forestall such trickery. She was willing to sacrifice her own marriage to her chosen husband in order to spare her sister embarrassment. Yet it would not be long before "she was jealous of her sister" (30:1).
It is not natural to be so giving and despite their truly noble intentions neither Sarah nor Rachel could be expected to maintain such lofty levels of selflessness. The Torah expects that man limit his kindness to that which can be maintained over time. "The Torah was not given to angles" but to those of flesh and blood and thus it is better to do a little less but to do so consistently, than to strive for greater but ultimately unsustainable heights.
The problem of tzarot is not limited to the inevitable fighting amongst the wives. The husband too will naturally gravitate towards his "favourite" wife leading to the situation of "when a man has two wives, one whom he loves and one whom he hates" (Devarim 21:15). Jacob "also married Rachel and he loved Rachel more than Leah". But soon, without Jacob even consciously noticing it, that extra love to Rachel that began as "more than Leah" became "G-d saw that Leah was hated and he opened her womb".
Sadly children are often the battle grounds used in such disputes and the names of the children reflect the increasing acrimony between these once beloved sisters. "G-d has seen my troubles" (Reuven) "G-d has heard that I am unloved" (Shimon), "Now my husband will become attached to me" (Levi). Not to be undone Rachel names her son Joseph because "G-d has gathered away my humiliation".
Tragically the problems of tzarot do not limit themselves to the protagonists themselves. Much of the fighting between Yosef and his brothers was a bitter reflection of the children of different mothers unable to resolve their differences. And even death often does little to limit the ongoing feuds. "And Rachel died " (35:19).. and Reuven slept with Bilha the concubine of his father and Israel heard (35:22). Reuven was a faithful son ready to defend the honour of his mother Leah.
Jacob had placed his bed in the tent of Rachel and upon her death Jacob brought his bed into the tent of her maidservant Bilha, seeing her as the link to his dear wife. Reuven could not tolerate him choosing Bilha, a lowly maidservant in his eyes, over Leah the matriarch of the Jewish people. He thus rearranged the bedroom furniture so that Jacob's bed would be in the tent of Leah. Reuven never actually slept with Bilha - something that would have been an unpardonable offence - "rather he moved around the bed of his father and scripture considers it as if he had slept with her" (see Shabbat 55a).
With great people a little sin is a big deal. What for Reuven was an act of concern for his mother is described by the Torah as an act of adultery. This, for better or worse, is the fate of the Jewish people. Our minor faults are viewed by the world as heinous crimes. While this may seem unfair it reflects the notion that "The Holy one Blessed by He is exacting with the righteous" and helps to put us on guard to try and correct even our minor faults. May we merit to do so. Shabbat Shalom!