If the trustor is now dead and you are referring to a trust that was revocable until the trustor's death, then yes. RCW 11.103.050, enacted in 2011, states that a contest must be commenced within 24 months of death, or within four months of the receipt of notice sent by the trustee, whichever is earlier.
If the trustor's is still living and you suspect that someone is unduly influencing him or her, then you should talk to an elder law attorney about the possibility of a vulnerable adult action.
Attorney King's answer is spot-on.
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