Definitions of ‘interfering’ are challenging. 

When you get to the other location, when you take an action to sign in, you should be prompted to do so, use the ‘other’ login.

When you’re at home, and take an action that requires a login, you should be prompted for your own data.

I’ve not messed with this in a long time, but that’s the way it worked when I did. Note that just ‘using’ a local Sonos doesn’t require a login, you just need access to the WiFi. You’ll only be prompted for a login when doing certain actions, such as adding a streaming connection. 

Yes, when you are at the other location, in the Sonos app go to Settings > App preferences > Reset app

You can then connect to the system at the other location, entering the username and password for that system, which will give you full control for any troubleshooting you are going to do.

When you return home, reset the app again and join your own system. Your system will not have changed in any way.

Actually when you return home you *might* not have to do anything - the S2 app seems to be getting better about switching Households with less pain.

If that is true, then there should be no need to first reset the app when joining the family member’s system in the first place, which I suggested.

I do have Sonos systems in different locations and when connected to Wi-Fi at the respective locations, when opening the Sonos app on arrival I can just select “Join existing system”. So, I do agree that switching Households is becoming easier in the app.

What I am unsure about is when the systems at different locations have different system owners. Maybe it is just as simple as Bruce ​(@Airgetlam) suggested, just sign in with the respective system credentials when prompted to do so.