While it has no legal standing, and a letter of intent cannot override a will, it can be an invaluable document for your family in any kind of an emergency, not just your passing.
Unlike a will, it does not need to be prepared by an attorney. It is, however, a repository for everything your family would need to know in an emergency situation. Just what needs to be included in such a letter?
With so much of our financial lives transitioning to an online presence, user names, passwords and PINs for accessing your accounts are essential. Names and contact information of bankers, advisors, attorneys and any other professionals who handle your assets will be important information. However, it is just as important to include a narrative of your wishes for things that are not covered by your will and don't require probate intervention to distribute. Perhaps you have some treasured collectibles or unusual items that your heirs might need your guidance on how to handle them. In fact, your will can point to your letter of intent as to the expression of your wishes for such things, so that updated or additional information can be added without changing your will.
For a more comprehensive listing of what you should consider putting in your letter of intent, check out this article.