juanmaria,
i love the mother in law story. i know exactly what you mean and have had many many similar experiences with older generation australians (including one whose relationship to me shall go unmentioned) who can't seem to grasp that "abos" is not an okay abbreviation. and there are millions more examples i'd be happy to go into some other time...
i think the changing of these kinds of words is a result of the (thankfully) changing attitudes. but i don't think it works the other way around. would it make any difference what words your mother in law used? i find that people who can't change their attitudes are also unwilling (unable?) to change the terms they use.