It would appear that the license plate is 1923 Indiana vintage so the ladies may not have been all that far from home to have visited Mammoth Cave. I wonder if they’re sisters, perhaps twins ?
Yes, I would guess sisters, love those outfits!
I’d lean toward a mother and daughter not sisters. The girl on the left looks like a teenager and the lady on the right, given the bags under her eyes and general build, I’d say she’s in her late thirties or a healthy early 40’s.