What kind of relationship exists between Nick and the Buchanans?
Nick is Daisy's cousin, and Nick identifies with Daisy because she used to be a middle-class Midwesterner. He is fascinated with and even envious of the status and wealth she has attained. Through what happens to Gatsby, however, Nick will discover Daisy to be a hopeless little fool and Tom a domineering brute.
Nick is also drawn to secrecy, and he is curious to see how the affairs of Tom and Myrtle and Daisy and Gatsby will play out. We see him attend three parties: Tom and Daisy's, Gatsby's, and Tom and Myrtle's. Nick cannot stand Tom: he calls him a brute, especially after he witnesses him slap Myrtle. Nonetheless, Nick reserves moral judgement.
Later, Nick will host the climax of the novel: Daisy's date with Gatsby. He is complicit in Gatsby's seduction and adultery, so his moral standards are no better than Tom's, really.
At the end, Nick will denounce the Buchanans, especially after they leave a mess to be cleaned up in Myrtle's death. Not to mention, they leave Gatsby to blame. They do not attend his funeral. Nick cannot stand that they hide behind their wealth and status. Before he leaves them and returns home to the Midwest, he says:
They were careless people, Tom and Daisy -- they smashed up things and creatures and then retreated back into their money or their vast carelessness or whatever it was that kept them together, and let other people clean up the mess they had made.