I find it pretty striking that God chose Sarah to be the mother of His people.
As a reminder, Abraham and Sarah thought they would help God out by adding Hagar to the wife-harem. She was pregnant and had a son, Ishmael.
In Genesis 17 God reaffirms His promise to Abraham. Of course, looking around at his situation Abraham points out that Ishmael would be a great solution - after all, that would make sense right? But God had a plan. His plan was for Sarah, Abraham's first wife, to be the one to produce the blessings promised to Abraham. Even though God had other options (of course God is God and can do anything), He chose Sarah to be the mother of His people.
I think this says a lot about the Lord's desire for his people. God didn't take the "easy" way out (by fulfilling His word through Ishmael), but instead took Abraham as an example to show us all what He was capable of. Sarah was nearly 100 when she finally gave birth in Genesis 21.
Even though Abraham and Sarah both kind of screwed things up by bringing in Hagar, the Lord still blessed them. He still used them.
I've been told a lot throughout my life that God has big plans for me. What are they? Well I've never been certain exactly. But what I do know is that I am here to do His whatever pleases Him to the best of my abilities. I've screwed things up too. But right here is proof that the Lord can use me anyway.
And the fact that Sarah was Abraham's first wife is not lost on me. I think the Lord placed her with the honor of being the mother of His people because of that. Similarly, Jesus ended up coming from Leah's children - Jacob's first wife (Genesis 29:31-35).
Our families don't always follow the Lord's plans for our lives. But that's OK - God can (and will) still work with us where we are at. His grace is far reaching. We just have to be willing.