Would it be legal to transfer the money in his account to ones personal account?
No, absolutely not. The proper process has to be followed and authorised by the court, you can't just transfer money to yourself.
Our fears are that the tax authorities will rip it off before it gets to the family.
The tax authorities won't rip it off. The person handling the estate has to distribute the assets according to the law, and recipients are responsible for paying tax on the inheritance.