Spent some time in Texas and Arkansas and heard Bubba for eldest sons and Sissy for eldest daughters especially with older folks. I’m 1/8 Japanese and grew up hearing my grandma calling her older brother “Nii-san” and never his name. Although for them it was more a respect thing instead of an affectionate nickname. I still think it’s cool to have nicknames that informs the listener the birth order of the siblings maybe because of how I was raised.