I think his hair too look the same with fl mother
so the duke is trying to say our empress was sleeping with her half brother?
idk, i was looking at previous chapters and the brown hair old man said the white hair knight was actually his nephew and not his son, but he could be telling that story in order to protect both his son and the honor of the empress's mother
he has blue eyes and light hair like the empress grandfather and i dont think he's like that because he's old. that's my guess anyway
my theory is that the white hair dude is the son of fl mother and brown hair knight since it seems she was hiding meeting her former lover even from her father