Iowa Code § 633.281: Interest of witnesses.

No will is invalidated because attested by an interested witness; but any interested witness shall, unless the will is also attested by two competent and disinterested witnesses, forfeit so much of the provisions therein made for the interested witness as in the aggregate exceeds in value, as of the date of the decedent’s death, that which the interested witness would have received had the testator died intestate. No attesting witness is interested unless the witness is devised or bequeathed some portion of the testator’s estate.