Try resetting the Sonos app under App Preferences and rebooting your phone.

Why not use their own phone.controller - at least that way you know that it will continue to work for them...

So - it isn’t supported, and I shouldn’t want to do it regardless?

What? Reset your app then connect to the system. As stated by @GuitarSuperstar .

It couldn't be much easier.  

Obviously it needs to be an S2 system if you are using an S2 app.

I mean, it’s not supported for the Sonos app to be aware that I, as an owner of a Sonos system, might also want to control another system that I don’t own when I’m at that location.  Wiping the app’s knowledge of who I am and what systems I own isn’t “supporting” this use case - it’s a work-around.  If that’s the case, so be it.  It’s not an awful work-around, even if it is pretty inelegant.  I just figured this would be a use case Sonos would want to support, especially because other support content implies that it used to be supported.

The app is just a remote control.  It holds no information.  Everything is held on the speakers. Your post is nonsense.  

I am not even convinced you need to reset the app.  The app should just offer to connect to the available system. But resetting the app will do no harm whatsoever.  When you reconnect to your home system everything will be as before.  The app just shows you what is there. 

Somehow this feels like the app is holding some information..  Maybe I’m just imagining things..

As stated above, this is when I’m on a Wi-Fi network with a functioning Sonos system - just not my home system.

Yes it will be holding a Sonos ID for your system, an email address and account password.  Nothing else that won’t be recovered when you log back in at home.  I can’t remember what happens if you click “let’s fix it”, but if that doesn’t result in your being able to connect to your in laws’ Sonos system, you need to reset the app.  You can then choose to ‘connect to existing system’ and connect to the in-laws’ system.

If you do have to reset the app, then when you get home and reconnect you will have to log in on the app before you can change any settings and manage the system.  But everything that matters about your system - playlists, streaming services etc - is held on the speakers.  The app just picks up the info from the system it is connected to.

This post may be of use to you more geneerally.