Here is a proof of concept. I'm simulating when the output of the (opamp used as a) comparator switches as a function of the resistance of the two "thermistors", R1 and R3.
Note that the two track each other, and to a first order, the comparator switches when the two resistances are equal. You could build-in a slight offset by slightly increasing or decreasing R2. The supply voltage is not critical; it could be anything from 5V to 12V. Lower might be better so as not to cause self-heating of the thermistors.
I didn't show how to switch a relay, but you could connect the OUT signal to the gate of an NFET, and use that to switch a relay...
I'm leaving it up to you as to which thermistor R1 or R3 is the hotter one. That will determine the direction you want to offset the trip point, and the polarity of the OUT signal...
The opamp/comparator could be an LM358 or a TL072.