What was the relationship between Abraham and Mary?
In her song, the Magnificat, Mary praises God for keeping his promise to Abraham and to his descendants. Luke 1:54-55
What the Bible says
Luke 1:54
He hath received Israel his servant, being mindful of his mercy:
Luke 1:55
As he spoke to our fathers, to Abraham and to his seed for ever.
Douay-Rheims Bible (Challoner revision), public domain.
What the Church teaches
Abraham is the model of the obedience of faith. The Virgin Mary is its most perfect embodiment.
Catechism 144 · Catechism of the Catholic Church