Well, there's a legal difference between accomplice liability for assisting in an actual crime (principal) versus liability for aiding a person after the commission of a crime (accessory after the fact). If a person assists in planning & carrying out a crime, then -- yes -- one is equally culpable as the perpetrator. This is why a get-away driver is as culpable as the actual bank robber.
The punishment for an accessory after the fact varies from state to state. The feds can punish such an accessory with a sentence up to half of what a principal could receive. 18 U.S. Code § 3. Here in Indiana, the level of punishment that an accessory after the fact may receive varies depending on the level of crime of the principals, and -- interestingly -- provides that parents, children, and spouses cannot be charged with assisting a criminal. Ind. Code § 35-44.1-2-5.