If avatar Aang is basically avatar Roku and Roku is Zuko's great grandfather does that not make Aang Zuko's great grandfather there for they are related?
No, each Avatar is their own person. While their souls remain linked in some fashion, their bodies are distinct, as denoted by the fact that each nation takes a turn as having an Avatar. If they were actually related by blood, merely wiping out the airbenders (as the Fire nation did) would not have stopped the cycle.
Well it’s kind of confusing, Aang technically IS Zuko’s great grandfather, but they’re not related. It’s referred to MANY times in the show that the reincarnation of the avatar ARE the same person, with A different form. This is shown with lines like “Our shared past” and “We are you”. So genetically no, Aang and Zuko aren’t related, but since Aang is Roku, spiritually he is his great-grandfather.