What matters most is whether the ship is actively piloted or not. More or less that means the pilot is on board, but can include when the ship is part of a party which has an actively-piloted leader. Person A pilots Ship A, whose owner has you listed as IFF Friendly, Ship A is also leading a squadron with Ship B (also owner has you IFF Friendly). Sensors of Ship A and Ship B are shared with your piloted ship (provided your ship is owned by You)Edited By: Kay Dallben on Year 18 Day 191 11:39
As you say, being in the faction does set you as IFF friendly unless someone in the faction manually added you as enemy. If the assigned ship has the same sensor power as this other ship, you don't get extra Sensors. But recall that the ship has to be actively piloted in order to share sensors, so if you hop from one ship to another the former ship is no longer piloted.
If you're convinced it's not working according to the Rules or as described here, gather the exact details of what you're trying (all ship types, what vision looks like from each ship, owners & pilots of the respective ships, all steps you take and results of same), and submit a bug report. Images help for Sensor troubleshooting.