Epimers are diastereomers that contain more than one chiral center but differ from each other in the absolute configuration at only one chiral center.
1 and 2 have the same molecular formula and the same structural formula and, therefore, stereoisomers. 1 and 2 are not mirror images of each other and, therefore, are diastereomers.
Diastereomers 1 and 2 differ from each other in the absolute configuration at only one chiral center. Thus, 1 and 2 are epimers.
In epimers the chiral carbon atoms whose absolute configuration makes the two compounds different are called the epimeric carbons.