I've been writing a mystery story recently, but I've run into a problem concerning the major plot twist, and was hoping for some guidance from more experienced writers.
Here's the scenario:
Person A (the narrator) and Person B are twins, and thus are very close. Recently, a series of murders has happened in the area, and the group of friends that Person A and B are part of are trying to find out what's going on.
Person C, one of Person A's friends, is convinced that Person B is responsible for it all. She provides compelling evidence that proves her claim, and says that they should confront Person B as soon as possible. However, Person A refuses to believe her and provides counterclaims that are equally as valid as Person C's.
The twist is that Person C was actually right, and Person A was blind to it because of how much he cared about his twin- not even considering the chance that Person B could have actually hated him (which Person B indeed does).
Of course it's more detailed than that, but I don't want to make it too complicated. I just want to know if this is flat out lying to the reader or an unreliable narrator.
Also, do any of you think this is a cheap twist?