Try using the Volume Limit feature, in each room's settings. It's actually a scaling factor. 

Thank you for the reply.  I don’t want to limit how high each room goes.  I want them to raise and lower proportionally and stay that way.  For example, if my family room is at 50%, my kitchen is at 25% because that difference between them allows for you to hear both.  If I raise the volume and the family room is now at 75%, the kitchen would be at 50%.  This does work this way, however when I close out the app and then go back in the next day, it doesn’t keep the speakers at their proportions.  It starts them all over again as equals and I have to rebalance them.  It’s annoying because it should stay where you set it like every other feature in the app and not reset them. 

The volume limit feature implicitly restricts the max volume, it’s true. However it can maintain the relative offsets between rooms in the background, allowing the sliders to be aligned. 

As for your observation that all the sliders are adjusted to be the same overnight, it’s nothing to do with closing/reopening the app. It sounds like you could have an alarm set for the group.

Or possibly are turning off power to the Sonos devices, I believe that causes a reset of the volume. 

No, reboots don’t affect the volume setting.

Does the group volume not do this already? Slide each room to desired volume, then the group will then adjust each relatively. Or am I missing something about the OP’s ask? 

You are correct that it already does it.  The problem is that it’s not keeping them there.  When the app is closed out and later reopened, it’s not keeping the volumes set to where they were.  It’s resetting them to equal volumes. 

Can’t say I’ve ever noticed that with mine. I tried quitting the app and when I re-launchd (on iphone). It was same as when I took the screenshot. 

The controller app simply doesn't do this. Something else is aligning your volumes.