out-laws for in-laws

I have heard the phrase out-laws (for in-laws) and even parents-out-law fairly often, but it is usually applied to parents of the partner in a long standing but non-legalised partnership. Which leads to me to the word question, why are parents of a spouse in-law? Are there, or were there once upon a time, rights and responsibilities, bestowed by this relationship? Inheritance, rights of succession and wardship for child brides are areas that spring to mind as possibilities.