Pirkei DeRabbi Eliezer
(Laban) took his two handmaids, and gave them to his two daughters. Were they his handmaids? Were they not his daughters? But according to the law of the land the daughters of a man by his concubines are called handmaids, as it is said, "And Laban gave to Rachel his daughter Bilhah his handmaid to be her handmaid" (Gen. 29:29).