selm

etiquette "rules" are not all black and white. A lot of them seem black and white because they are accepted by most people as the tried and true way to handle a particular situation. They are common sense guidance for handling people interactions in large or small situations. The main purpose is to make interactions kinder for each other.

For your specific example it really is a matter of "whatever works for all". She may feel(as well as your mother- ask her) as a guest in your house she should not assume where you want the glass. (When I was little we were taught to put things on the counter next to the sink. This way you didn't have to remove everything from the sink to wash them). When you go to her house, as a younger, more agile person it might be nice to help put dishes in/near the sink per her wishes. So - do whatever works best.