In 1993, my father wrote a note stating he was giving my sister the house that sits on an acre of land. The note was notarized but he did not give her a deed for the house.
In 2000, my father had a will drawn up that do not speak of this house but states that all the property is not to be divided and if the siblings decides they want to sell some of the land, then they must all be in agreement.
Is the piece of paper considered a legal document?

The piece of paper that your father wrote in 1993 might be considered a legal document depending on the surrounding circumstances. For example if your sister moved into the house or started making improvements to the property after he wrote that note then a court might find that it was a valid and enforceable contract. If so then this note would take precedence over the terms of the will.

If the note said that your sister was paying him anything for the house and property, or if the note said that he was giving it to her out of "love and affection" or anything like that, then a court would probably find that it was a valid and enforceable contract.

The note itself is not a deed unless it contained the names of the seller (your father) and buyer (your sister), some sort of legal description of the property (such as the address), some type of purchase price (even just $1.00 or love and affection), and if it was attested to in addition to being notarized.

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The note itself is not a deed unless it contained the names of the seller (your father) and buyer (your sister), some sort of legal description of the property (such as the address), some type of purchase price (even just $1.00 or love and affection), and if it was attested to in addition to being notarized.

Check the requirements for deeds in your state. They may not require a recitation of consideration or attestation.