During Ordinary Time the two readings are not designed to comment on each other. A particular book of the Bible and a Gospel are chosen and then read through chapter by chapter. Is it a coincidence or a providence that both the Old Testament lesson from the Book of Tobit and the Gospel text from Mark mention a woman being married seven times? In the Bible and today being married seven times seems extreme (just ask Elizabeth Taylor). The point of both readings is that even in extreme circumstances God is still at work. When you know the power of God, Jesus said, you understand that no matter how things look, God can make a way. Like Sara just spread out your hands and pray.