Hi,
I'm designing a power system for a car.

The alternator is connected parallel to the batteries (4 x 3.2 V in series) and the lights/E-Sus/Daq loads. After the main fuse, there's a wire which controls two relays which will turn the Daq and E-Sus loads off if the alternator fails or the main fuse breaks. That way, we can save the batteries to power only the lights.

My question is that if the main fuse breaks, will the p-mosfet highlighted in yellow prevent current from flowing backwards from the batteries (green arrow) which will turn the relays on?

Thanks,

 

No, the battery voltage will turn on the MOSFET.
Read here: https://forum.allaboutcircuits.com/threads/replace-schottky-with-mosfet.122090/

 

Ditch the mosfet and just use a Shottkey diode..

 

if the main fuse breaks, will the p-mosfet highlighted in yellow prevent current from flowing backwards from the batteries (green arrow) which will turn the relays on?

As noted, no.
A MOSFET conducts equally well in both directions when the gate is biased on (as it is when the battery is connected.
Use a Schottky diode instead, as Dd suggested.