Boy, a lot of people here have simple family relationships. Siblings do not have to have the same last name. Names can change between birth and later life. Lots of people and cultures use the terms Uncle and Aunt quite liberally without the need for a genetic relationship.
The question of Beru and Owen’s last name was set back in 1977. It has been Lars since the beginning. So the question of Lars vs. Skywalker has always been there. Luke calls them aunt and uncle. That is meaningless. That is what he calls them. Giving Anakin a step-brother in the prequels just answered that longstanding question of how is Owen his uncle. There were many theories floating around before AOTC answered the question. The extent of all we got in the OT was some vague hint that Owen was upset that Anakin went off following Kenobi.
Just a few items that I have in my own family history. A couple adopt a baby. It is actually her nephew, but they give the child their name and raise him as their own. He knows his full siblings, but thinks they are cousins and doesn’t find out until he is in his 80’s that they are his siblings. A man has 5 daughters, his wife dies shortly after and the youngest daughter is taken by the wife’s family and raise in another state. He remarries and about a decade later his daughter marries his 2nd wife’s next to youngest brother. That creates some interesting relationships, and the step mother shuns the being called grandmother and instead is called aunt. Another, a man accuses his wife of having an affair because she hid her second pregnancy and subsequently divorces her. She changes both her children’s names. As an adult, her son marries a businessman’s daughter and his father-in-law takes him under his wing and endows him with a title that can be used as a name. Consequently his daughter has a different name on her birth certificate and her driver’s license. Trying to figure out genetics or family relationships from names is often hopeless.
I the old days before the PT, we knew nothing of Luke’s mother. Before ROTJ, we didn’t even know he had a sister who had vague memories of their mother (which only makes sense after the PT if Leia is considerably stronger in the force in the non-Jedi areas than Luke). Before that, we don’t know if Owen or Beru are related to Luke’s father or mother. But the mother must not have been from Tatooine if Leia ended up the adopted daughter of Bail Organa and with the title of Princess. So that kind of cut off any relation through their mother to either Owen or Beru. And Lucas never answered that until he scripted AOTC. Whether he had decided that before that is open to debate. The link to Kenobi was always rumored, but that doesn’t help figuring out the family relationships with Luke.