As if you could forget, Ross and Carol divorced just before the show began, after she cheated on him with her best friend Susan. If Monica and Ross weren't brother and sister, Heller's theory goes, his friends would have used this split as an excuse to gradually distance themselves from the whiny, insecure palaeontologist, as people often do eventually let their friendships fade from one half of a split couple. Without Monica, he wouldn't have become. If his friendship with college buddy Chandler fell apart, he wouldn't have grown close to Chandler's roommate Joey. Come to think of it, Ross has so little in common with Phoebe and Joey, their friendship seems hard to pin down anyway. Of course, this would probably also meanon the day of her own failed wedding.