Most state's probate laws provide for varying degrees of information available to an estate's interested parties, like beneficiaries and creditors. Before you take the time to learn more about what rights you have as what I presume is a beneficiary, you should understand clearly that the executor's attorney represents the executor -- not you. In an ideal situation, it wouldn't make a difference, and information would flow as freely as trust and communication. Sounds like your situation is a little less than ideal. The executor's attorney is either protecting his or her client or trying to save the estate some money.
Was an inventory filed by the executor, listing the assets on hand at the start of the administration? There's your starting place. Some states grant beneficiaries the right to demand and compel accountings that show any changes to the assets since the administration was opened. These typically become available after certain times, and so it will matter how long this matter has been going on.
Finally, and I suppose practically, would your next step change if you found some irregularity today as opposed to the near future? It sounds as though you'll get what you're after, even if it is on the executor's timetable (which may be their right, by the way.) Be vigilant, but use this time to get your ducks in a row regarding what you know, so that you can compare it with what you're told when an accounting is provided to you.
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 not rely on this answer alone and nothing in these communications creates an attorney-client relationship.
It is common in this area to finalize an Estate with a Family Settlement Agreement. Most attorneys will attach a First and Final Account to the Family Settlement Agreement which should itemize all the assets administered as part of the Estate, including detail on whether the asset was sold and, if so, for how much, as well as all the expenses paid. Much of this information is also available on the Pennsylvania Inheritance Tax Return, a copy of which is filed with the local Register of Wills. You can go to the Register's office in the county where the Estate was opened and review the return. You are well within your rights to demand a formal accounting but must recognize that it will substantially delay when the Estate is finally distributed. I am only 20 minutes away if you would like to discuss this in further detail.
NOTE: Mr. Fischer is an attorney licensed to practice in Pennsylvania. He can be reached at 610-269-0900 Ext 2 or firstname.lastname@example.org. This answer was prepared for educational purposes only. By using this site you understand and agree that there is no attorney client relationship or confidentiality between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed attorney that practices in the subject area in your jurisdiction and with whom you have an attorney client relationship. Frequently the question does not include significant and important facts and time lines that if known could significantly change the response.
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The answer to this question is found in PA law. If this was CA, the accounting would need to be filed with the court BEFORE the court orders "almost the entire estate". After reviewing the accounting, you could file objections to the accounting asking for documentation. The matter would need to be resolved BEFORE the distribution.
Your siblings should contact the local bar association and ask for a referral to a local probate attorney. The cost for the consulation may be divided amongst all of you and you would know how to proceed and protect yourselves from an executor you don't trust.
I am only licensed to practice law in CA. Any statements made are general answers intended as opinion only and may not comport with the laws outside of CA. A local attorney practicing this area of law should be consulted to provide you with the law and options that may apply to your specific question.