I'm not going to dodge the premise of your quandary, which is that accountability and responsibility are necessary components of a civilized society, and that idealizing anonymity also introduces problems and is prone to abuse.
Your example is disconnected from this premise though, for other reasons. Even with air-tight biometrics, determining the exact identity of which twin crossed the border, the police would still have their hands tied.
Biometrics would not solve a hit-and-run manslaughter, simply because the level of surveillance necessary to prove the identity of every passenger in every driver's seat at all times would amount to oppressive totalitarianism.
In reality, a rental car provided to a vactioning traveller could have been stolen for a joy ride, and possibly returned before the renter noticed it was gone. The police would need a stroke of luck, in the form of some video footage or a witness further corroborating the identity of the driver at the time of the incident. A border crossing, and a rental agreement wouldn't confirm the criminal, even if a jury might convict on that basis alone.
Anyway, back to accountability. There's a fine line painted at the boundary of the statement: Those who have nothing to hide, have nothing to fear.
It's always going to come down to the constituents of a given society, to determine the level of discipline required to participate with adjacent members. An individual's neighbors will determine how volatile their environment is. A permissive environment may attract those rejected from other regions. A fortress may sterilize all behavior. Everyone should be able to choose their own peers, according to preference, but what if one finds themselves chosen for the purpose of exploitation? There's no easy answer to this situation. Complex systems require defense mechanisms to persist and withstand chaos. My point, though, is that people should understand which side of the defense mechanism their on, and whether they are participating deferes, or regarded as the defended, or perhaps if they are the repelled chaos.
