Sandra Day O'Conner marries William Rehnquist in the 1950's

She almost certainly would not be on the Supreme Court. She'd be a trivia point: "You know, Mrs. Rehnquist also actually went to Stanford Law School - a real pioneer."
 
I don't know if there's any actual legal barrier to spouses serving on the court together, but she quite likely would not have been nominated if for no other reason than the optics. Or at least, not at the same time as she was nominated IOTL.
 
Well, assuming Rehnquist himself still roughly follows the same career path, if she feels there’s no chance she’d make it on the court she might instead choose to stick to politics (she was after all a member of the Arizona Senate).

As for the court itself, again assuming only minimal butterflies before 1981, her seat is instead probably filled by Cornelia Groefsema Kennedy, also considered by Reagan before he picked O’Connor.
 
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