NAFLD is usually caused by obesity. However, it is also a common finding in those with ApoB defects. People with these defects have a curious phenotype: normal or even fantastic lipid levels but with NAFLD/NASH. Essentially, the defective ApoB doesn't get secreted from the liver so serum cholesterol levels look good, but the liver becomes fatty as it cannot export TG and cholesterol.

Certainly not trying to diagnose your familial disorder over the Internet, but taking a trip to a good lipid clinic might be something worth considering if you wanted to be aggressive in the pursuit of a diagnosis. (Which, again, might not be what I'm talking about above.)