In the legal custom of that day a barren woman could give her maid to her husband as a wife, and the child born of that union was regarded as the first wife’s child. If the husband said to the slave-wife’s son, “You are my son,” then he was the adopted son and heir. So Sarai’s suggestion was unobjectionable according to the customs of that time. - Ross
 Ross, A. P. (1985). Genesis. In J. F. Walvoord & R. B. Zuck (Eds.), The Bible Knowledge Commentary: An Exposition of the Scriptures (Vol. 1, p. 56). Wheaton, IL: Victor Books.