Did you mother-in-law also leave a will or is everything she owned in the trust?
A Form 1310 will have to be filed, regardless, unless there is a court certificate or your mother-in-law has a surviving spouse.
Your sister-in-law is the trustee of the trust, but the trust document is not considered a court certificate. Your sister-in-law also appears to be the proper party to file the final return (and any other returns or documents that are required to be filed) unless there is a will in which an executor is named.
Frankly, you need to take this one step at a time and, if you have the least bit of doubt (or the assets are substantial), you should engage the services of local legal counsel and/or a local tax professional.
