As the other answers state, the grandparents do face some risk of getting in trouble if they allow him to live with them while aware that he has a warrant for his arrest. They should consider talking to a lawyer for themselves, and encourage the nephew/grandson to contact a separate lawyer for himself.
In a nutshell, yes. If they became aware of the warrant today they should respond promptly. All persons who are charged with crimes are innocent until proven guilty, but it is also pretty typical that a person facing serious time is, in their own mind, innocent of the charges.
But being on a "most wanted list" is not the same as a warrant. A warrant tells law enforcement that this person has failed to appear before a judge when ordered to do so. If your nephew failed to appear before a judge, that is not the same as admitting/denying a charge.
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His grandparents could potentially face charges for rendering criminal assistance in the first or second degree if they know he's wanted but harbor or conceal him. See RCW 9A.76.050, .070, and .080 for details. His grandparents need to hire a lawyer and consult with him or her about the best course of action to protect themselves.