(decedent's will left everything to niece, but if she didn't survive, then equally to niece's two children in trust; niece died in 1995 and decedent hospitalized in 2007; niece's husband procured new will from decedent within two months of decedent's death that gave niece's husband and children money outright and named husband as personal representative and an interest as a contingent residuary beneficiary; trial court applied doctrine of partial invalidity to eliminate husband of niece as administrator and residuary beneficiary; on appeal, court affirmed with result that one-half of residuary devise passed under intestacy law and that residuary devisees were tenants in common in residuary estate).