NASA astronauts Mark and Scott Kelly are identical twins and scientists took the opportunity to do extensive tests with them. One pair of twins is not conclusive but the study is interesting.

The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight https://www.science.org/doi/10.1126/science.aau8650

NASA Twins Study Confirms Preliminary Findings https://www.nasa.gov/feature/nasa-twins-study-confirms-preli...