Online family history data is categorized as "intellectual property," that is intangible property. Intellectual property can be bought, sold, rented and of course, inherited. When a person dies in the United States, the property he or she owns instantly passes to a legal entity called the "estate." The subsequent ownership of the property is determined by the inheritance laws of the individual states in effect at the time of the death. Individuals can determine to some extent the disposition of their estate by creating documents such as wills and trusts. Each state also has court rules, in addition to the inheritance laws, that determine the procedures that need to be followed to dispose of the property.
Not all property has to pass through probate. If a person transfers ownership of the property before death, the property is not considered part of the "estate." This can be accomplished by either selling the property or transferring it to some sort of legal entity such as a trust or a company. Some people avoid this whole issue by having their company (corporation, LLC etc.) or a trust own all their property during their lifetime. Unfortunately many people do not consider all of the information they have online, including family history data, to be property and they neglect to transfer that type of property before death.
Online intellectual property (including genealogy files) create some practical as well legal issues. Most genealogy websites have logins and passwords. Many of these websites also have restrictions concerning revealing those passwords to anyone who is not the registered owner. It is a really good idea to keep a list of all of your passwords in a safe place with your other legal documents. Many people involved in the area of "estate planning" recommend keeping these valuable documents in a safety deposit box. Apparently those who make such a recommendation have never had the experience of trying to get access to a safety deposit box after a person has passed away. The dead person's legal representative may have to go to extraordinary lengths to get access to the safety deposit box because most financial institutions will lock up the safety deposit box until receiving a formal court order appointing an administrator or personal representative of the estate.
I believe that some types of valuable personal property are properly kept in a safe deposit box: network passwords and logins are just not that type of property.
The next important issue with intellectual property in an estate is the determination of whether or not it has any monetary value. This can be very hard to determine. It may be that the genealogy files contain information that is valuable to genealogists but the files may not have any commercial value. Another consideration is whether the heirs acquiesce to the wishes of the deceased. It is not uncommon for the heirs to disagree over the provisions of a will or trust. That disagreement may extend to a dispute over who "owns" the online genealogy files.
All of these considerations strongly indicate that if you are involved online to any extent, you need to make sure that there is some concrete provision made for the disposition of the online files in the event of your death. If necessary, it is a good idea to consult with an attorney or other competent advisor.