If the Estate is still in probate or if probate has not been completed, you need to file a license to sell with the probate court and to have this approved by the court.
There are two ways to have this approved: either have all of the heirs/siblings sign assents, or just file it (giving notice in accordance with the probate rules) and have a hearing.
Attorney Myers is not quite correct. The only way of knowing what your rights are is by reading the will and seeing what powers it grants you. There's a distinction between leaving the real estate to the heirs subject to a sale, leaving it outright, and lumping the real estate with the residue.
Then you need to see whether the will waives the requirement for a license to sell. If it doesn't, you need to petition the court for a license, with either the consent of the heirs or formal notice if you don't have everyone's properly signed consent. Whatever the case may be, the property CANNOT be sold unless you know what your rights are under the will and can give clear title.
I'll just add onto Attorney Golden's answer.
Even if the will gives a power of sale, you still may want to consider a license to sell, especially if there is potential for strife with the siblings. A judge signing off on a power of sale is awfully hard to overturn and may prevent people from holding up the closing.
You mention in your question that the house is now on the market. If you get a buyer, and need to close promptly, it is possible to convey title to the property without getting a license to sell (or a 'right to sell' as you refer to it) from the Probate Court.
You didn't explicitly say that the will has been probated (proven) by the Probate Court, but for the sake of discussion, I assume that it has, since you did say that you are a co-executor.
Massachusetts law has a very peculiar way of looking at any real estate that becomes part of an estate. To quote an authority (Newhall) wiser than I: "unless the will provides otherwise, the executor has nothing directly to do with the real property.” The title vests immediately in the heirs if there is no will, or passes to the devisees [devisees = the people named to take the property in the will] if there is a will.
Since you have 9 sellers, all of them would also have to sign the Settlement Statement at the closing, to document that they agreed to the distribution amounts, and to the expenses being paid out at the closing. Really cumbersome, but still possible. The buyer's attorney would insist on all the signatures.
As the other answers explain, you still have to satisfy the Probate requirements pertaining to the estate. You didn't mention whether the power to sell real estate is written into the will. I'm thinking maybe it is not written there, since someone has just questioned you about "right to sell."
You did mention that the death was two years ago. Massachusetts law shuts the door on most creditors 1 year after death. So, if you have settled all legitimate perfected claims against the estate (a topic for another day), you would not need the proceeds from the house sale to satisfy the legitimate estate creditors.
The buyer's attorney would be checking your probate court file to confirm that title to their purchasing client's new real estate is not encumbered by an unsettled debt in the estate file. The buyer's real estate attorney would insist that the file be clean before there could be a closing.
So, your question has as much to do with real estate law as with the law of probate and the law of wills.
Ultimately, the person you have to please is the real estate buyer's attorney, and they operate under title standards that everyone else follows.
You are smart to be asking the question now, because if you get close to closing time and you have not straightened out the probate file, and negotiated an advance agreement with all 9 (!) devisees, you will cause a lot of inconvenience and hardship for everyone involved in the transaction. A good real estate broker will see these needs in advance and include something in the Purchase and Sale Agreement that solves the problem.
There are many people involved in your transaction. Maybe instead of asking "Do we need a license (probate court authority) to sell real estate" you could be asking:
"As an Executor with so many beneficiaries looking at me, should I make sure to get a license to sell real estate, in order to protect myself?"
Your Probate - Real Estate attorney can make sure that you are covered, and make it possible for everyone to smile as they walk away from the closing table.