What was the relationship between Simeon and Mary?
Simeon blessed Mary and Joseph. To Mary he foretold that a sword would pierce her soul. Luke 2:34-35
What the Bible says
Luke 2:34
And Simeon blessed them, and said to Mary his mother: Behold this child is set for the fall, and for the resurrection of many in Israel, and for a sign which shall be contradicted;
Luke 2:35
And thy own soul a sword shall pierce, that, out of many hearts, thoughts may be revealed.
Douay-Rheims Bible (Challoner revision), public domain.
What the Church teaches
The Catechism teaches that Jesus united his Mother, more closely than anyone else, to the mystery of his suffering.
Catechism 618 · Catechism of the Catholic Church