The "Affiant" is the one swearing under oath that they have personal knowledge of the facts stated in the affidavit. This cannot be the person who inherits. You qualify as "disinterested". But, if you don't have personal knowledge of the facts, you can't swear that they are true. If you make a false statement under oath, it is a crime.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
See Texas Probate Code Ch. 48, et seq: http://www.statutes.legis.state.tx.us/Docs/PB/htm/PB.III.htm#52A. Section 52 provides a guideline.
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It is important that you have personal knowledge of the facts since you are swearing to them under oath. You should be a disinterested party for the purpose of this affidavit since you have no beneficial interest in the money.