Michael and Diane Keaton are not actually related, and oddly enough neither of them was born a Keaton.
It's understandable that one might think they're related; they're of the right age to be brother and sister. And in fact, though only six years separate them, Diane was already an established actress when Michael was picking a stage name, and as the story has it he stole it from her.
Perhaps steal is a little strong though. Michael had been working as a comic for a few years under his birth name, Michael Douglas. But as there was already a Michael Douglas in the film biz, he had to choose a new name to be able to join the Screen Actors Guild (SAG rules prohibit two members having the same name). The story goes that he had been reading an article about Diane Keaton the morning he chose his new name.
As for her, Diane Keaton was born Diane Hall, but when she was getting into the biz there was already a Diane Hall registered with the Actors' Equity Association (the stage version of the SAG, and which observes the same name rule). She opted to switch to her mother's maiden name, Keaton.
One of Diane's best-known roles was in Woody Allen's signature film, "Annie Hall." Allen, her boyfriend at the time, is said to have named the character and film after her (her nickname is Annie).
As for the Quaids, they are in fact brothers. Though Dennis is the more established of the two, it's in fact Randy who's the older brother.
Perhaps that has more to do with Dennis's classic good looks than anything though. Randy has been working for longer, having gotten his start in the 1971 classic "The Last Picture Show." A long list of increasingly big film roles got him onto the "Saturday Night Live" cast in the '80s. Since then the films have gotten increasingly bigger, but the roles haven't. Throughout his career he's favored supporting roles, a decision that's left him now as a well-respected comic and character actor.
His little brother Dennis went the other way, becoming a leading man pretty early on in such films as 1983's "Jaws 3-D" (an unassuming start, to be sure) and "The Right Stuff."
In 1987 he starred in "The Big Easy," the New Orleans-set noir film that solidified his status (for a while anyway) as a leading man.