An element A decays into element C by a two step process: $A\to B{+}_{2}H{e}^4; B\to C+2e^{-}$ . Then

${}_{Z}A^A\to {}_{2}He^4+{}_{Z-2}B^{A-4}$

${}_{Z-2}B^{A-4}\to 2{}_{-1}e^0+{}_{Z}C^{A-4}$ 

$\therefore$ A and C have same atomic number. So, A and C are isotopes.