Native

As the space ship appears to be traveling in normal space (rather than some kind of hyper drive or warp drive and such), under relativistic speed, the two scientists would experience time dilation, meaning time will pass much slower for them than the people back on whatever planet they launched from. Their twin counterparts would have died of old age while little time had passed on the space ship. Even instantaneous communication would be slowed down by the speed of thought being dilated too.