OK, there it is. Why do we call our SO’s parents “in-laws”? I know, I know…because it’d be too improper to call them outlaws! 
But seriously, what’s the origin of this phrase? And, for extra credit, how do they refer to these people in other languages and/or cultures? - Jinx
Just going to WAG the first part and say that it’s because they become your “mother/father” by the law of marriage.
In Afrikaans:
Skoon- (lit. clean)
Skoonma = MIL
Skoonpa = FIL
Skoonsuster = SIL
Swaer (lit. heavy) = Brother-in-law.
etymonline gives in-law’s first appearance in the thirteenth century and says the “law” is Canon law, which “defines degrees of relationship within which marriage is prohibited”.
So, apparently, your mother-in-law is your mother according to the Bible and you can’t marry her. The real question is, who would want to?