How to address you mother in law:
This is a tough one. I’d say the problem of how to address her is not nearly as big as the problem of how many stamps do you put on? I mean, I know it goes by weight, and how exactly are you supposed to know her weight? Ask her? No, there’s no doubt that would end up in an underestimate and the deadly Return To Sender!
Unless your name is not Sender, in which case you have nothing to worry about.