What was the relationship between Leah and Jesus?
Leah was the mother of Judah, and Judah is an ancestor of Jesus. Genesis 29:35; Matthew 1:2
Matthew’s genealogy does not name Leah. That she is an ancestor of Jesus is inferred from two verses.
What the Bible says
Matthew 1:2
Abraham begot Isaac. And Isaac begot Jacob. And Jacob begot Judas and his brethren.
Douay-Rheims Bible (Challoner revision), public domain.