Time for the Stars (1956) – The twins, Tom and Pat, are discovered to have a telepathic link, which makes them suitable as communicators for voyages of interstellar explorations. Pat stays home and grows old creating a wealthy family; Tom goes to the stars and explores until newer, better ways are found. This is a fictional exploration of Einstein’s Twin Paradox.
For more information on Heinlien's Juveniles (written between 1947 and 1958) see our entry on Robert Heinlein's Rocket Ship Galileo, The Literary Encyclopedia profile by William Patterson, University of California, Santa Cruz
Patterson, William. "Time for the Stars". The Literary Encyclopedia. First published 10 November 2004
[https://www.litencyc.com/php/sworks.php?rec=true&UID=16614, accessed 20 March 2018.]