How long is saliva viable?
Saliva buffer usualy provides stability at RT for month or even years. Genotyping companies like 23&amp;me use collection kits like Norgen's (https://norgenbiotek.com/product/saliva-dna-collection-preservation-and-isolation-kit) which allows to store the sample up to 5 years. 

So my advice would be: get the spitting vial at your place, send it as regular mail to your brother, and then ask him to send it back to you once filled with saliva. By doing so you won't have to worry about temperature.

Sending fresh saliva should be ok if your brother sends it refrigerated (4°C). I keep fresh cells up to at least one week in the fridge in proper medium, and saliva is a pretty decent buffer and has some antimicrobial properties.