Since the probate has started, you can notify the court (should be included in the court record) that you believe you have a legal claim. The inventory date is the date by which a lawyer has to produce a list of total assets discovered. I believe the 18 month closing date is the longest that probate goes on. I dealt with something like this in the case of my father's estate, and when the probate extended past 18 months, probate had to be refiled (that costs money, and in my case, all entitled parties forked over a percentage).
You can also get in touch with the attorney handling the probate and I believe they will give this information to you. I am not sure they are required to by law.
My lawyer provided folks with information about the death and heirs named.
As an aside, if the assets discovered are not complete, check your state's
unclaimed property department. It is usually listed in the site map for your home state or the decedent's state government online. If nobody is aware of an asset it will pop up many times on that site. I actually discovered an asset the court and lawyers were not aware of. I then was required by law to notify the probate lawyer and the court. I found out that the time a lawyer puts into discovery is many times determined by their level of professionalism. Duh, some lawyers are not professional. I had to fire one before I found a good and thorough one.
Also, now thinking about it, I believe that anyone named in a will is entitled by law to a copy of the will. It should be sent to you with a letter of explanation. When I was executor of my dad's estate, I had to make sure that was done.