I don't see this as any different from when a JW parent cuts a DF/DA child out of their will and leaves the money to the WTS. Legally, you can leave your money to whoever you wish. Ideally, you'd want to continue your legacy by passing things on to your children. But, obviously the relationship was strained and thus the conditions. Was their being JWs the cause of the strain? Likely but it could have been more him than them. Really difficult to tell. At least he left things to his relatives and not to some huge non-profit organization.
It's a shame that religion is such a divisive influence. Probably the most divisive in human history.