As someone that grew up partly in Southern Illinois with lots of relatives in Southern Indiana I can tell you they have a great deal in common. Both regions are very "southern" culturally, with very distinct accents.

Illinois is an interesting place as it features large changes in culture from north to south. I was born in Northern Illinois and lived there until I was 10 when I moved 5 hours south. There is an enormous cultural difference. As the map shows Northern Illinois is part of the "Midlands" with a flat/generic accent whereas the Southern Illinois/Southern Indiana accent sounds a lot like Woody Harrelson's (who was born in Texas). The greater Chicagoland area is its own thing, the map shows it part of Yankeedom but I disagree - I lived in Chicago twice in my 20s and I've lived in Yankeedom (Massachusetts) for 25 years now and I don't see much similarity. I'd group far northern Illinois, Wisconsin, Michigan and Minnesota in their own group, maybe called the "Opers"

Yeah southern Indiana, Illinois, Ohio, northern Kentucky, and western Tennessee, the Ohio River Valley in essence, are culturally very similar and "appalachia-light".

But IMO that's a very far cry from eastern/south-eastern Kentucky, eastern Tennessee, western NC, WV, etc; truly core Appalachia.

Its also a far cry from mid-Indiana, which is culturally identical to northern Indiana, most of Illinois, Iowa, southern Michigan, maybe even as far as southern Wisconsin, Kansas, and Nebraska. That's Corn/Rust belt, very crop-farm oriented, dairy farms, extremely flat, but not as rural as many people think, not nearly as rural as Appalachia or the west.