How are enantiomers related to each other?
By reflection in a mirror plane.
Enantiomers are non-superposable mirror images. In organic chemistry, for this to occur, it generally means that a given carbon atom is bound to 4 different groups, i.e.
Note that enantiomers have identical chemistry in an achiral environment. As soon as another optical isomer is introduced, the relationship between an enantiomer and the ADDED optical isomer becomes diastereotopic, which may allow separation and different chemistry.