I'm Lucy, and I'd be happy to answer your questions today. I'm sorry for your loss.
The answer lies in how the property was titled before your father passed away. If your parents held the property as "tenants by the entireties" or "joint tenants with a right of survivorship," that means your mother became the sole owner of the entire property at the instant of your father's death. Property held in either of those ways is NOT part of probate, and therefore, the will does not control - the will only determines how probate assets are split up. Most married couples purchase property as tenants by the entireties.
On the other hand, if they held the title as "tenants in common," there is no right of survivorship, and your nephew would now own your father's share. If you don't know, check the deed. Also look for any limiting language in the will that says, for example, your nephew only gets the house when the second spouse passes away. That's fairly common.
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