Throughout the Civil War, Kansas remained a Union state. Though there was a large pro slavery faction in residence at the time, Kansas never joined the Confederacy.
I know it is TNAOS canon that Kansans speak with Southern accents, but very few actually do. This is not a southern state.
Unless the Harrises proudly trace their line to pro slavery ancestors it's unlikely to hear Rachel calling Lois a Yankee. I know she is supposed to be southern but that makes it prejudicial instead of just a casual remark.
Y'all I can live with. It's better than you'uns, which I occasionally hear young people saying these days.