Jamieson, Fausset and Brown on Genesis 30:3-9

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.