I'm not so certain he made them wealthy. It was Jim's wife's family that inherited his estate (she died less than a year after he did).
Actually, Jim did leave everything to Pamela Courson. She was a junkie and died 3 years later so the rights to Morrison's work went to Pamela's father. Jim's parents sued him many times over the years to get their share of the estate. They finally won which did make them wealthy, but Courson is still the executor.