I am a Robot Programmer in the Auto Industry and am on 10 years now.

The upper tenant has complained to me multiple times about it being too hot in his unit because the bottom tenant does not actively manage the thermostat.

@Svend W. the most practical solution to be 99.9% sure is to have a small electric heater on a thermostat.

Boilers don't run continuously--when the thermostat calls for heat, the boiler fires up. 

WiFi thermostat.

It will take you 2 days max if you are a regular formula programmer or know what you are doing to build a schedule, build folder system, and build a good SKU list.

So, we know that we want to at least add some zoning control so that both tenants have some level of control over the thermostat.

My wife being a computer programmer helped.My daughter went from struggling in high school to doing well at the Univesity of Arkansas. 

Hire a good house keeper and between VRBO and the house keeper 85% of your running a short term rental are taken care of.You could hire out the lawn care too if your brother doesn't want to deal with this.I use wifi thermostats to save on electricity when no one is at property and a lock box for keys.I see you say you don't have a pool.

Rently can be nice if you set it up to include the smart lock and smart thermostat.