My name is ***** ***** I am a NSW Solicitor. Thank you for your question, and will do my best to assist you with your question. Please understand this is not legal advise Please understand this is not legal advise but a guide to assist you.
The only way to protect the estate for your child and grandchildren is to see if the wife will enter into a Binding financial Agreement where she gives up the right to any inheritance that your son receives.
If she does enter into this then it is protected.
it is a pre-nup but in Australia under the Family Law Act they can be entered into at any time before or during the Marriage.
If she is not prepared to do this then it is going to be near impossible to protect the inheritance if it goes to your son.
If you change the Trust and leave it all to the grand children then she cannot do anything to make a claim but it will cost you money because you will be removing your son from the trust and that is a re-settlement and you will pay stamp duty on the 20M the trust is worth.
Here is some information about Binding financial agreements.
I hope this makes sense and is of assistance. If there is nothing further
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