My advise is threefold.
1) Start now while they are both 'compos mentis' of sound mind.
2) Contact a soliciter who specializes in family law.
3) Get both of them to make wills, even if they are husband and wife.
To help you with No 1, find out where they keep all the bills, insurances and any other documents you require. I was lucky in that my mother in law kept very good houshold accounts going back years in two large suitcases so when she moved from sheltered accomodation into a nursing home we kept them and she just collected all the bills etc and gave them to us and we took care of them.