My first rule is that unless the infraction is serious (as in causing injury, blood, fire, etc.) I don't take second-hand reports. It just sets up the situation of giving them power over their siblings to say "DS did this or that" and nobody like a tattle-tale. Mine especially like to gloat about their brother getting in trouble, so we try to minimize that. When they do, the gloater usually gets the same consequence as the troublemaker.
If I know (or strongly suspect) they did something, I don't ask and give them the opportunity to lie. It is just too hard at that age for them to overcome the impulse to save face or avoid consequences by lying. If they insist on lying about it, I just tell them "I know that's not true. Stop lying." If they continue to argue they can spend time in their room.
Mama to three boys ('03, '05, '07)