The fiduciaries are responsible for filing a return for a trust (Form1041) if the trust meets the threshold for filing.
See https://www.irs.gov/instructions/i1041#en_US_2021_publink1000285943
When a trust terminates a final return is typically required to be filed (which is a good practice, regardless).
See https://www.irs.gov/instructions/i1041#en_US_2021_publink1000286084
You should also be aware that two types of special needs trusts exist, first-party and third-party, and this appears to be the latter. As such, the trust could have grantor trust status depending upon certain terms in the trust that might have been reserved to the grantor (your father-in-law) when the trust was created.
You should seek professional guidance from a local tax professional.
See https://taxexperts.naea.org/listing/service/estates-gifts-trusts
