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Not entirely same user case, but spending alot of time in the wilderness I use a combination of two apps.
Locate Driver --> [link]
which sends coordinate sms reply at keyword.. txt keyword, get reply...simple and helpful.
which is a widget placed on homescreen. You start widget, set receiver number and schedule. Widget also holds "I'm OK" and "I'm not OK" shortcuts.. simple and helpful too.
I've also read Whatsapp and other social apps can report positions, obviously using data, not sms. Maybe the best solution in urban areas.
But you probably want something that works in the background, and your brother will not know its running and no email/sms traces in phone log? Sorry, thats way over my head.
Best of luck
Maybe Google Maps has the needed feature...
I track my father with Locate driver.
Its for auto answer some info about user via keyword in sms. Maybe worth to check out.
Its a different approach to it, but I'm happy.