What does Yu-Ching Fei's joke 577 1 1 mean?

The son said to his father, "I like uncle Zhang's daughter very much!" " "Dad secretly told him," that's your half-sister. You can only be friends. " The son added, "I like the girl next door to Uncle Li, too! Dad added, "That's your other half-sister. Don't tell your mother. " The son told his mother these experiences in tears. The mother comforted her son: "You can find whoever you like. Don't be afraid, you are Lao Wang's son!