father-in-law = step-father?

The step- prefix is from (13^th-century) Old English steop-, referring to bereavement: steopcild was not stepchild as we know it today, but rather orphan before we acquired that word from Latin via Old French. A step-parent used to be the adopter of an orphan.

In-law is from the same time period and refers to Canon Law, which established relationships (and thus allowable marriages, etc.) on the basis of consanguinity and adoption. The two terms could presumably have been used interchangeably until step- lost its connotation of orphanage, which happened as late as the 20^th century.

The OED has a seventeenth century citation from one of Fletcher and Massinger’s plays illustrating the use of step-father to mean father-in-law, with the suggestion that it might have been a conscious misuse.