I'm looking for some advice on a pre-nuptial agreement for a second marriage. My mum is getting married in a few months time. Unfortunately her soon-to-be husband has a terminal illness so the issue of wills and future planning has taken more importance than would normally be the case for such a happy occasion 
She basically wants to change her will to say that, in the event she dies before he does she would want him to be able to live in their house (that she bought and paid off the mortgage for before they met and that is in her sole name) for as long as he lives/wants to. But that when he dies she doesn't want his children to have any claim on 'her' house. She isn't being cruel - his two children are provided for in his will with 'his money' and her house is her legacy to her children (i.e. me and my sister).
If she writes this in her will is it sufficient or does she need a pre-nuptial agreement?
Thank you for any advice 