genesis 30:3 And she said, Behold my maid Bilhah, go in unto her; and she shall bear upon my knees, that I may also have children by her. - Free Bible Online

      3-9. Bilhah . . . Zilpah--Following the example of Sarah with regard to Hagar, an example which is not seldom imitated still, she adopted the children of her maid. Leah took the same course. A bitter and intense rivalry existed between them, all the more from their close relationship as sisters; and although they occupied separate apartments, with their families, as is the uniform custom where a plurality of wives obtains, and the husband and father spends a day with each in regular succession, that did not allay their mutual jealousies. The evil lies in the system, which being a violation of God's original ordinance, cannot yield happiness.

JFB.