it is a wise child that knows its own father
It is difficult to truly know who one's father is, especially in the case of a child born to unwed parents. A: "Ben was actually raised by his stepfather and didn't know it until years later." B: "Wow, I guess it's true what they say—it is a wise child that knows its own father."
It is a wise child that knows its own father.
Prov. You can never have certain proof that a certain man is your father. (Implies that the child in question might be illegitimate.) It is a wise child that knows its own father, but Emily is so much like her dad that there's very little uncertainty.