Since your sensor produces a sine wave as a signal I would propose for the conversion part, to do the following:

Drive a cheap 4046 PLL with the sensor signal. From the VCO control voltage drive a buffer op-amp (optional but suggested) and then the A/D of your PIC. Now you have a clean analog voltage that follows your flow converted to a digital.

If you want to do the opposite, drive a voltage to the VCO of the 4046 and now you have a voltage to frequency converter (very linear I might say).

Ioannis