There are clear situations where two or more entities are related parties. As I've mentioned before, two companies that did business together where a husband and wife were respectively executive directors of the two companies.
When John Wardle was chairman, he was a related party to City. NYCFC is a related party, as are the other CFG clubs. Basically the definition is where a person, or close relative of that person is related to the reporting entity, including being part of a group of companies or a joint-venture.
But there's a subjective element to it as well. Who exactly is "a close relative" for example? What does "related" mean in practice?
The situation is that Sheikh Mansour is Sheikh Mohammed's brother, and the latter is the ruler of Abu Dhabi, which has a state-owned airline. But there's a number of questions to be answered to establish whether there's a related party relationship between any of those and City.
Sheikh Mansour and City are almost certainly related parties, as he's the ultimate controlling shareholder. But it still might depend on how much day-to-day influence he has over us. Maybe not that much but let's assume he is a related party.
Are he and Etihad related parties then? Almost certainly not. Although he's a member of the ruling family, and his brother is the ruler, it's hard to see how they could be seen to exercise influence over Etihad's day-to-day decisions. The Open Skies case document seemed to be pointing out to Sheikh Mohammed that the Etihad sponsorship of City was mostly covered by central funds. The inference in that case is that he didn't know.
City have never declared Etihad to be a related party, and that's been (presumably) agreed by the auditors. But the PL lawyers and other experts may take a different view, as UEFA's representatives apparently did in 2014. The relevant accounting standard (IAS 24) is clear up to a point, but can be interpreted very subjectively after that. So it's one view against another, pretty well similar to someone saying one player had a good game and someone saying they didn't.
But, to make the point yet again, it's irrelevant whether Etihad is or isn't a related party as long as their sponsorship is deemed to be market value. I still think the PL have picked a strange hill to make a stand on here.