The later executed (signed) Trust would now be the valid trust. Most properly drawn trusts and Wills will also contain language which revokes all previous Trusts or Wills, so that there is no confusion. It is not uncommon for a surviving spouse to either amend or have a new trust drafted after their spouse has passed away.
First an attorney must determine whether the old trust was revocable by the surviving spouse. To the extent it became irrevocable, your father in law may have violated it to the extent he took assets out of the old trust and transferred them to the new trust. Second, an attorney should determine whether your father in law actually transferred assets out of the old trust and into the new trust. If he left assets in the old trust, merely signing a new trust will not cause the assets in the old trust to be subject to the provisions of the new trust. Also, the old trust probably has very specific rules about revoking it, amending it and/or taking assets out of it, so an attorney must determine whether your father in law followed those rules in removing assets.