You don’t need any legal documents, but a written agreement or memorandum of understanding with your father-in-law’s wife might make sense just to make sure everyone is on the same page and she does, indeed, contribute your father-in-law’s retirement income.
Harry S. Margolis practices elder law, estate, and special needs planning in Boston and Wellesley, Massachusetts. He is the founder of ElderLawAnswers.com and answers consumer questions about estate planning issues here and at AskHarry.info.