If the burial plots were purchased and paid for, and the coffin was also, the real property has not been abandoned.
An argument can be made that the relatives showed no interest in the coffin for years, until they heard that money was going to be made on it. That would probably weigh in favor of it being abandoned property.
Also, at least in the rental business, you are usually only required to store unclaimed property for a certain length of time, and after that, there is going to be some procedure for the possessor to dispose of it, if it isn’t claimed.