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 destined to be one of jealousy, bickering and even hatred. Jewish law, recognizing this sad situation, disqualifies the testimony of one of the tzarot regarding the other—we are afraid that they will simply lie.

Despite the best of intentions to make a "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 her 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 (Breisheet 16:6) claims, to the ongoing persecution of Sarah’s descendants by the descendants of Hagar. Rachel, our Sages claim, suspecting that her father Lavan was about to try to 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" (Breisheet 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 angels", 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 battlegrounds 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 outdone, 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 alone. Much of the fighting between Yosef and his brothers was a bitter result of the children of different mothers unable to resolve their problems. And even death often does little to limit the ongoing feuds. "And Rachel died (Breisheet 35:19)… and Reuven slept with Bilha the concubine of his father and Israel heard” (Breisheet 35:22).
Reuven was a faithful son, ready to defend the honour of his mother Leah. Jacob’s bed had been placed in the tent of Rachel; and upon her death, Jacob moved his bed into the tent of her maidservant Bilha, seeing her as the link to his dear wife. Reuven could not tolerate Jacob 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 may never have actually slept with Bilha. “Rather, he moved around the bed of his father, and Scripture considers it as if he had slept with her" (see Shabbat 55a).
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 be 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.