"Disinheriting" means leaving a valid will that deliberately gives nothing to a descendant or spouse. And yes, it's legal.
You can contest the validity of the will, but not because it leaves you out. You would have to prove, most likely, either undue influence or lack of capacity. Undue influence means that someone else really wrote the will, that it wasn't what the deceased wanted, even if the words came from him. It's hard to prove. Lack of capacity is even harder to prove, because for a will, capacity only means knowing one's property and the "natural objects of one's bounty" such as children. As one law school casebook entry pointed out, a man can have a habit of standing in the middle of the road half-dressed, at night, shouting back at thunderstorms, and still have the capacity to write a valid will.
If there are significant assets involved, you should talk to an Indiana wills and estates lawyer, who can give you reliable advice based on all of the facts of your case.
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