Before moving from Genesis 16 it must be mentioned that Abraham was commanded to take Hagar to wife.
D&C 132: 34-35
As I ponder this- I feel this is an extremely hard trial and I am thankful that I am not asked to do this in this dispensation.
Some scriptures came to my mind as I pondered this difficult situation of Sariah, Abraham and Hagar :
1 Nephi 11:17
And I said unto him: I know that he loveth his children; nevertheless, I do not know the meaning of all things.