Hallway light switch makes bedroom light brighter
I think you have a multiwire branch circuit (also called shared neutral). Instead of running two separate cables from Phase A and Phase B at the panel to the branch circuits, the electrician runs a single 3-wire cable carrying Phase A Hot, Phase B hot, and Neutral (shared) to power two branch circuits. Saves a cable run, and therefore labor and copper costs.
The downside is that if the loads are unbalanced on these two branches, there can be enough voltage drop in the neutral conductor to cause slightly unbalanced voltages at your devices. Perhaps that's why the bedroom light brightens when the hallway light comes on. But it should take a bigger unbalance than that, so I suspect you may have a corroding connection in the neutral line somewhere. Get a voltmeter and do some testing.