Commento su Genesi 30:4

ותתן לו...לאשה. she provided Yaakov to be Bilhah’s exclusive sexual partner.

את בלהה שפחתה לאשה, “her servantmaid Bilhah as wife.” This was similar to what Leah had received from her father when she had married. She also gave that servant maid to her husband Yaakov, as reported in 30,9. We know that Joseph in his early years kept company with the sons of Bilhah and Zilpah who are described there as his father’s “wives.” (Genesis 37,2) This teaches that these women were not concubines, but regular wives with all the financial security that such a status guarantees the wife. None of the 12 “tribes” were born to women who were merely concubines. The only time when the expression “concubine” is used about them is when they are mentioned in comparison to Rachel and Leah, when the indiscretion of Reuven is alluded to by the Torah in both Genesis 35,22, and 49,4. Gad and Asher had already been born when that indiscretion took place.