New Zealand Law
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The position at law remains the same as when your father first passed away. The executor of his estate is the person who has the responsibility for the headstone. Unfortunately if she has neglected this, you would need to see if she is prepared to pay or consent to replacement. Although you don't get on with her, you would need to see if she was prepared to do anything. If she refuses, then you could take action yourself. You should first contact the organisation who looks after the cemetary-likely to be the local council, who usually look after cemetaries. You could then arrange for the new headstone to be put into place
When someone dies without a will, normally the surviving spouse is appointed the executor, called an administrator when there is no will. It is not automatic, but very common. She would in any event be entitled to most of the estate.