I cannot speak for OP's sister, but I personally feel gut problems in my back. There is no medical explanation for that, that I am aware of. But when I eat certain foods, I have intense back pain the next day.

The only reason I think it is related to gut problems is because the back pain is perfectly correlated with certain foods (e.g. large amounts of gluten, smoked meat) and has 0 correlation with normal reasons for back pain.