Can a will in probate for 18 months suddently be considered invalid?

Advising you properly on this matter would require a consultation and a review of the documents. From what I am gathering, you need to be careful. Making distributions that may later be found incorrect can result in a liability to you as the person who acted as executor. If the will does not sufficiently address your mother's specific intentions, then it is likely that intestacy laws will govern the distributions. Formality is required in wills to avoid situations such as these. Based on the information in your question, the proper heirs may be your brother and you regardless of the will, but these details should be more carefully addressed based on a full understanding of the family background.

I also encourage you to prepare notes today of everything you recall about the legal advice you were given, what you understand has been done, etc. Do not throw away any documents. Please be sure you have afforded your attorney every opportunity to explain where things stand and how he advises you to proceed, but also prepare yourself in the event you have exposed yourself to liability based on attorney advice or inaction.

You and your attorney need to have a heart to heart talk. If the will fails to specify in writing how the property should be distributed, then the property will have to be distributed pursuant to the laws of intestacy. Assuming that your mother died without a spouse and you and your brother were her only descendants, you and your brother will divide her NET estate equally. Since your mother's instructions were not in writing, the law of intestacy will control, not her oral instructions.

Since you have already distributed property to your brother and his children, you and your attorney will need to determine whether you have properly distributed one-half of your mother's NET estate. Hopefully, you and your attorney have handled the creditors of the estate properly because you could be held personally liable for the payment of debts if you have wrongfully distributed assets of the estate to your brother that should have gone to the payment of creditors.

Finally, on the challenge of the will, the general rule is that your brother had 42 days to challenge the will after it was admitted to probate. Assuming that you and your lawyer gave your brother proper notice, his time to challenge may have expired. So, the will is valid to the extent that it named you as executor.

I advise you to have this heart to heart talk immediately. Best of luck.