The #1 thing that reduces WiFi range is interference from other WiFi signals, followed by concrete walls. If the cabins are off in the woods, then there shouldn't be other WiFi interference and thus a mesh system might work, provided the cabins don't have concrete walls.
How does your wife's aunt and uncle currently get WiFi in their cabin? Cable, fiber optic, satellite? If cable, you can just split the cable outside and run that cable to the other cabin, but that cable will need to get inside grandma's cabin. If grandma's cabin doesn't currently have cable going into it... then that's going to require professional installation, unless there's a garage that has electrical power you can run the cable into.
But for the time and effort it would take to dig a 200' ditch to bury the cable... why not just get internet installed at grandma's cabin by the same provider at her aunt/uncle's cabin and put it on her aunt/uncle's bill?