In Death of a Salesman, are Charley and Bernard related to the Lomans?
In a word, 'no'. That Biff and Happy address Charley as 'uncle', and Bernard greets Willy in the same way seems to imply a familial relationship, but their use of this term of address is the one sometimes used in a number of cultures by children for a male friend of their parents. That being said, it is abundantly clear that the relationship between Willy and Charley is more than mere neighbourliness. Although Willy died in the fond hope - recalling the death of his ideal salesman, Dave Singleman - that hundreds of former friends and business associates would flock to his funeral, only Charley attends, eulogizing the man and his dreams. In Willy's last nightmarish months of 'earning' straight commission, Charley safeguards his appearance as the breadwinner of the family by paying him fifty dollars a week, but at the same time repeatedly offers him a job. Charley is a true friend, in fact the only friend Willy has.
I don't believe that the Lomans in Arthur Miller's Death of a Salesman are genetically related to Charley and Bernard. As best I recall, they're neighbors, with Charley's son Bernard having grown up and gone to school with Willy's two sons.
The enotes study guide seems to support my suspicions: Charley is described only as "Willy's only friend," and Bernard is described as "the son of Charley, Willy's only friend and supporter outside of his family."
Their lives are certainly enmeshed (e.g. Charley continues to "loan" money to Willy), but as far as I recall, they're not literally related.