E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Stepfather and Father-in-law.
The stepfather is the father of one bereaved of his natural father by death. A stepmother is the mother of one bereaved of his mother by death. A stepfather must be married to a widow, and thus become the stepfather of her children by a previous husband; and a stepmother must be married to a widower, and thus become the stepmother of his children by a former wife. Similarly, stepson and stepdaughter must be the son and daughter by the father or mother deceased, the relict marrying again. FATHER-IN-LAW and MOTHER-IN-LAW are the father and mother of the wife to her husband, and of the husband to the wife. Similarly, sons-in-law and daughters-in-law are the sons and daughters of the parents of the wife to the husband and of the husband to the wife. (Anglo-Saxon, steop, bereaved.)