Didn't mention what your air travel plan was or whether you're flexible enough to use Seattle rather than Vancouver airport. Only three major airlines into SGF: American, United & Delta.
American goes TVR-DFW-SGF; United goes TVR-DEN-SGF. SEA gives you more options and is also cheaper if you end up paying. Delta via ATL is a lot of backtracking but viable.
Nearest airports if not flying into SGF in order: Branson, Fayetteville, AR (both on SW Air) ; Tulsa; KC; STL.
If you need a car anyway, Fayetteville is shortest drive and scenic. Tulsa is easiest as its a straight shot on I-44, airport is on SGF side of town vs KC or STL which are north side.
You can also take Greyhound from Tulsa to SGF: cheap, 3 hr drive with only one stop (Joplin) but would need pickup or taxi like you would at airport to get to aunts if she doesn't drive.