I finished watching The Complete First Season of Cheers on DVD and I noticed that in Part II of the episode Showdown Sam Malone's brother Derek tried to take Diane on a trip to Paris but when she was about to leave Sam and Diane got into a bickering arguement about not wanting to go to Paris because Sam thought his brother wanted to take Diane with him on a trip as a way for them to try to get married. Did Diane still remain best friends with Sam's brother in the other episodes of Cheers or were they no longer friends as the show continued?
Bestie

We never see Derek again, and I believe Sam only mentioned a brother once more (later in the series, when Rebecca mentioned her overachieving sister).