The order of people in documents vary by document. For example, in the US Census (and most if not all state censuses), the order is generally: male head of household, wife of head of household, children of head of household from oldest to youngest (but not by gender), stepchildren, other relatives (grouped together in the same order), other residents of the household (boarders, servants, etc).
If a census (that doesn't give relationships) listed, say, James, Frida, Joseph, Rebecca, Emily, and you knew that was a couple and their two daughters, but you didn't know who Joseph was, you could assume he was their son, and older than his sisters. It's not proof, but it's strong evidence.
The Kentucky Tax Lists vary a lot depending on year. In some years they only listed white men over 21 who owned a horse. In other years, it was more people.
It does appear that a section on the tax lists would be for a head of household and properties related to him/her. So it's not a coincidence that James Jr., Joseph and William have the same last name (like it might be with neighbors in an apartment building in the census). The joint names vs the slight separation may have to do with who owns what property, together or not.
But, no, even in cases where a son would be listed before a nephew, you have no way of knowing if Joseph is a son. The fact that he's listed with man with the same name (plus a Jr.) of the head of household is a clue that Jr. is a son, but you don't know if Joseph being listed first has to do with his ownership of property or his relationship to James Sr.
I can imagine a situation where two brothers equally inherit a property but one of them later dies. The dead man's oldest son then inherits the property. Now on tax records the joint owners would be the living brother and the son of the dead brother.
You need more evidence.
