Telepathic twins communicate across the stars

Question

The story starts with a space agency on earth looking for and testing twins. It seems that some are gifted in a way that allows them to communicate with each other even if they aren't speaking. The main character and his brother are such gifted twins and they are placed into a space program where one will stay on earth and the other will go into space.

As the story progresses, the brother who stays on earth ages faster then the one who went into space and creepily enough, when the space brother returns he ends up getting married to his great-great-great niece.

Answer 1

Time for the Stars, by Robert Heinlein

from the summary:

Travel to other planets is a reality, and with overpopulation stretching the resources of Earth, the necessity to find habitable worlds is growing ever more urgent. With no time to wait years for communication between slower-than-light spaceships and home, the Long Range Foundation explores an unlikely solution--human telepathy. Identical twins Tom and Pat are enlisted to be the human radios that will keep the ships in contact with Earth, but one of them has to stay behind while the other explores the depths of space.

also

Tom, however, returns to marry his last telepathic partner, his own great-grandniece, who has been reading his mind since childhood.