Isomer:
An isomer of a molecule have same number of atoms of each element but have their different arrangement. That is two compounds will have the same molecular formula but differs only in their arrangements.
So for C₂H₅OC₂H₅:
its isomer will be one which have same number of atoms but only differs in their arrangement that is C₄H₉OH. As this compound have same number of atoms as C₂H₅OC₂H₅ but only differs in their arrangement.