Let me throw a curve-ball, just for the spirit of discussion (and there has been some SPIRITED discussion here lately!): let us say that the 'younger brother' is learning-impaired (I personally believe this) and susceptible to yielding to pressure, false memories, coerced confessions, etc. SO- why has he not simply confessed to the whole affair, guilty or not?
Because they were pressuring him to implicate his brother, so that's what he "confessed" to. Zach was the suspect, Zach was the one they were going after. Remember the Knox case? They believed her boss was in on it because she sent him a text saying "see you later" and didn't understand that that colloquialism simply means "goodbye" in English. So they pressured her to implicate him and she did. Same deal here. He just acquiesced to what they were pushing for.