Slight variation of an LDO output
If your sensor output is ratiometric, then you should be using a ratiometric ADC to measure it (i.e. using the same power rails as a voltage reference for your ADC).
If your sensor is not ratiometric, then you should be using a fixed voltage reference for the reference of your ADC.
In your case, it sounds like you are using a ratiometric sensor with a fixed voltage reference for your ADC.
If your sensor is supposed to not be ratiometric but is significantly influenced by power supply fluctations nonetheless, then you should be able to use the same fixed reference ADC to also sample the voltage supply simultaneously with the sensor to know how much correction to apply.