Her code allowed no question that a father by blood must have a claim above that of any foster-father. (2.19.44)
Nancy believes that Godfrey has a greater claim on Eppie than Silas, even though Silas has been raising her for sixteen years. The claims of blood stem from a much older tradition than the family of choice that Silas and Eppie have created, but the novel doesn't end up supporting them. For all that tradition is so important in Silas Marner, Eliot doesn't seem to think much of it.