The rules for Sensor Range seem pretty straightforward, but they create some really odd situations.
It's quite possible for weapons like turbolasers to take out fighters without the fighters even being able to detect what's firing at them. Further, they couldn't even communicate with the vessel firing on them - but it can communicate with them.
If the PCs have a ship with Sensor Range Medium, they should always detect incoming fighters before the fighters can see them. At this point, they should almost always be able to evade the fighters if they wish since movement (Fly/Drive) is relative to another unit. If they can't detect you, then moving toward you in space is pretty damn unlikely unless you can't maneuver or you let it happen.
I really hope that sensor feeds can be shared (like a Star Destroyer keeping its TIEs aware of a target outside of the fighters' own Sensor Range) but this depends on Comms functioning so long as either unit has the range (otherwise the TIEs could only receive or only transmit - still not sure which one the Comms range covers).