It's a tough one.
If you take the 'I am really disappointed with you, that one who has done something wrong is going to get his brother into trouble as well. Because if no-one owns up, i will have to punish both of you, the innocent as well as the guilty.'
If you say that in a calm voice without shouting, I'm tipping that neither of your children are hard-faced enough to see his brother take the blame for something he hasn't done. Stare at the oldest, and I'm betting it will show in his face. Ask him if he did it - if he confesses, he may cry.
You have to explain that this is an imporant lesson - you can never lie your way out of trouble, you can only lie your way into bigger trouble.
I have no idea how much discipline your boys need - I've got girls, different approach - but make a punishment to fit the crime, and afterwards, remind him again that lying does not pay, and he should always own up.