What legislation or act in Australia covers wills and/or contracts in Australia?

In relation to:

Mrs Tilly was a frail, 80-year old woman who used to live on her own. She was worriedabout her ability to look after herself in her reclining years, so she asked her daughter and son-inlaw whether they would move back from the United Kingdom where they lived and worked, and stay with her and look after her until she died. She said that if they did that, she would leave her house to them in her will. They agreed. The daughter and son-in-law gave up their well paid jobs in London, took their children out of their school and sold their house and belongings. They moved into Mrs Tilly's house and looked after her until she died, but they then found that she had not left her house to them in her will.

Do they have a legal action for breach of contract?