Formal validity — s.15 gift to witness's spouse

A man duly executes a will that complies with s.9 of the Wills Act 1837. One of the two attesting witnesses is the husband of his niece. The will leaves a legacy of £20,000 to the niece and the residue of the estate to a registered charity. After the man's death, the niece seeks to claim her £20,000 legacy. The estate is solvent and there is no challenge to the validity of the will as a whole. What is the effect on the niece's legacy?