Difference between only signing on a stamp paper and signing on a stamp paper with notary signatures

I have some agreements I signed with my brother - all on Stamp Paper - I have dispute with him now:

• An agreement with him with the title "Affidavit" signed by only me (no witness)
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  An agreement with him with the title "Affirmation" signed by only me(no witness)

It all concerns flats owned by our parents.  No will is there. I now find that the agreement is not notarized something else was notarized that has no value to me.

Is there a difference in an agreement with real estate involved in it if  it is notarized or not? And it is only signed by me and he never signed it but he used it and benefited from it. It is also not registered.