Don't understand this joke?

When I saw this version, I accompanied my girlfriend for a marriage check-up. The doctor was my mother-in-law. She was shocked. Aren't you the child who was lost in Uncle Zhang's house five years ago? You have a birthmark on your back, and that child has one! Alas, poor boy, you came back when I threw you so far away!

It means that my daughter hasn't contacted her family for many years and won't come back until she gets married.