Ms. Sinclair makes an excellent point. Being nominated to serve as the Executrix is not the same as being appointed to serve as the Executrix. Knowing the status of the proceedings will go far to determine what your rights to information look like at this point.
The Clerk of the probate court in the county of your grandfather's residence can at least give you some idea of where the proceedings are, if they've even been initiated.
If your sister-in-law hired her attorney and filed the Will as early as the day after your grandfather died, several things could have happened by this point. A brief hearing would likely have been held as early as the end of October. This would be the hearing to admit the Will to probate and appoint your sister-in-law to her nominated position. After that, she would have 30 days to notify creditors generally, by publishing a legal notice. Your sister-in-law would also have 60 days to notify each beneficiary specifically. You would be entitled to a copy of the Will and the Order admitting it to probate.
The 90-day time that you cite is in fact the due date for the Inventory, if it is not extended by the Court. That 90-day window begins running from the date the Will is admitted to probate and the personal representative appointed. Even if your sister-in-law filed the Will almost immediately, this 90 days would only have recently expired. If she's actually overdue on the Inventory (which is filed with the Clerk, and not necessarily delivered to each beneficiary,) the Court will begin by sending her a reminder letter and may ultimately revoke her Letters Testamentary if she avoids this obligation.
In all likelihood, your sister-in-law was nominated to serve as the Independent Executrix. The word "independent" has significant meaning, and bears some important consequences for you as a beneficiary. Ordinarily, in an independent administration, your right to an accounting of the executor's actions does not mature and become enforceable until 15 months after the date that the Will was admitted to probate. Two years after the Will is admitted to probate, you could file a request for the executor to partition and distribute the estate. The result is some rather great leniency and freedom granted to independent executors.
In the meantime, while you may not be able to compel an accounting or force distribution, you can certainly remain vigilant about your sister-in-law's execution of her obligations. Her attorney does not work for you, and I think that you've gathered that. If you are less than satisfied with the information that you are receiving, if any, I would encourage you to retain your own probate attorney. Even if their only job is to keep an eye on your sister, your attorney could certainly help you keep her on the right track.
Cooperation and communication are key here, particularly when it comes to dealing with items of personal property that might not be of significant monetary value. An attorney on your side can help you present your concerns in a practical and hopefully non-confrontational manner.
This answer does not constitute legal advice. I am admitted to practice law in the State of Texas only, and make no attempt to opine on matters of law that are not relevant to Texas. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should never rely on this answer alone and nothing in these communications creates an attorney-client relationship.
What is not clear in your question is the status of any probate action. Being named Executrix in a will is just a nomnation by the decedent. In order to have the legal authority to manage the estate (and sell anything) a probate action must be filed and the appointment of the Executrix confirmed by the court with the issuance of Letters Testamentary. One thing you can do today is contact the court in the county where your grandfather lived and inquire regarding any probate action. If it has been filed, you can purchase a copy of the pleadings filed. Whether is has been filed or not, you need to consult with an experienced probate attorney who practices in the county where your grandfather resided. and you should do so without delay. It could be that your grandfather left only a few items of personal property of only nominal value, but as a beneficiary under a valid will, you are entitled to know what comprises the estate.
Best wishes for a favorable outcome, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.