Money received from inheritance, gift, or bequest - this would include grandpa's money - is considered separate property in Washington.  If you keep it seperate and do not commingle it with other joint assets or funds, it should remain defined as seperate. 

The court can order that seperate assets go the the other spouse, however, there generally has to be a really good reason for the court to do so.  Consult with an attorney to see if there are any facts in your case that would make the court want to award your seperate funds to your wife.