Filtering Reverse voltage from a coil
you just need to check whether the relay coil is energized or not. you can go for a simpler solution rather using mechanical relay (relays require higher power and it has limited life time).
let me give you three solutions.

Solution 1:
Opto coupler based: MCT2E(DC input)


solution 2:
Opto coupler based: MOC3021(Direct AC input)

Solution 3:
Transistor Switch based:

solution 1 and 2 gives isolation between the 26V AC input and your arduino

solution 3 doesnt give any isolation

you can select any solution based on your requirement

please wait for the schematics