I think it goes back to how you were raised. As kids we always had to pick up after ourselves and had to dishes after every meal. So now, regardless at home or at friends, I always take my glass, dishes, whatever, to the sink and volunteer to clean dishes and the kitchen. I think it is only courteous, but maybe your mom-in-law was raised with guest being treated like guest. I agree with the others that say don't sweat the small stuff, you'll be less stressed!