The writer-director brother act is relatively new in Hollywood history. Before the Coens emerged in the 1980s, the the only fraternal writing team I can think of were the Epstein identical twins (Casablanca). (There were acting teams like the Marx Brothers, and lots of brothers in various roles behind the scenes such as the Warners and the Selznicks.)
Since then, there have been frauteurs like the Farrelly, Wachowski, Wayan, Hughes, Weisz, and Polish Brothers. My guess is that the modern writer-director job often tends to be too hard for one individual to do, so brother pairs have flourished.
On the other hand, this trend may be dying out. I'm not sure if many new Coen-like brother acts have emerged in the movie business since early in the last decade -- perhaps because the end of the Baby Boom in 1964 reduced the average number of brothers the typical guy has.
But the question remains: do brothers who work together tend to get along better or worse?
There are a lot of examples in popular music history of brother acts -- the Jacksons, the Osmonds, the Everlys, Van Halen, the Kinks, the Beach Boys, Creedence, Oasis, Allmans, AC/DC, Bee-Gees, Radiohead, the Blasters, Dire Straits, Toto (who used to play in my baseball league at the park), the Dorseys, and so forth. (There might be an even higher proportion of sister acts, but I'll put that aside for another time.)
Many of these brothers squabbled something fierce, but then most musical acts do, so it's hard to tell whether brothers get along better or worse. The Van Halen brothers seem to get along better with each other than with their bandmates (which isn't necessarily saying much in absolute terms), while the Fogertys of Creedence got along worse.
My guess would be that show biz, with the seeming arbitrariness of fame, is even more destructive of fraternal comity than most occupations.