Why did Sarah allow Abraham to sleep with Hagar?

Sarah (Sarai) had not been able to conceive. And even though God had told the couple they would eventually have children, Sarah was impatient. She may have even doubted the chances of having children at such an advanced age. And so she advised Abraham (Abram) to sleep with her maid, Hagar, hoping she would conceive and give them a child. Once Hagar fell pregnant, Sarai was jealous, and later even told Abraham to cast out Hagar and her son Ishmael so that he would not share in the inheritance of her son Isaac.

Not only did Sarah allow Abraham to sleep with Hagar, but she also encouraged it! She was the main reason all of this happen. According to the law, a child her maid conceived with her husband would be her´s. Some would say, in her age, it was a pragmatic decision, but from the perspective of faith, Sarah didn´t have enough of it.