Why is someone a 'Step Mother' Instead of a 'Mother in law'?

I think they should have reversed that and made 'step mothers' the 'mother in law' and vise versa..
Who comes up with these terms?
OK, it makes sense to me. BUT as a rule if it makes sense, then the people who make decisions usually don't go with it.
And I don't really know how long the term mother in law and step mother have been around, but it was decided way back when.
Ha ha. Good point. Never thought of that before.
WISH I KNEW WHO CAME UP WITH THE TERMS BUT COULDN'T FIND IT.
I DO HOWEVER AGREE WITH YOU TOTALY.
a 'step mother' is the wife of one's father.

a 'mother-in-law' is the mother of one's spouse.


Answers:
I completely agree, some people that answered don't understand that you KNOW what they mean, just that they should be reversed. I think if someone had a father that remarried, then she should be your Mother-in-law, like the person before me said, 'mother-by-law', that counts here too, because she would only be your mother by law. Good question!!
step mother would be your fathers new wife while a mother in law would be your spouses mother
Marry your Dad and she will be both.lol
Yeah, that's a really good point - it makes more sense.
They call your dad's new wife a Step Mother because you'll be stepping on egg shells around her until you're 18 and/or she'll be stepping all over you, forcing you to do all the really gross chores around the house until you're 18.

They call your husband's mom a Mother-in-Law because if you follow your emotions and kill her, the law will be after you.
interesting
but i think if she is ''STEP'' or ''IN LOW ''they the same ''LOVING MOTHERS''
ha,ha
step mom is when ur dad marries another woman and mother in law is when u marry somebody and her/his mom is ur mother in law :) (mother BY law) get it?
it doesn't really matter. there are more important things to worry about like what i am having for dinner.