It is unclear what you mean by "in the event of a divorce or an estate," but marital property division agreements have to be in writing to be legally enforceable. Oral evidence may be relevant to determination of a just and right division of property by a court presiding over a divorce case.
This general opinion DOES NOT create an attorney-client relationship. No communication can ever replace the specific advice of a lawyer. Each person's situation is different, and additional facts can change legal outcomes. You should consult privately with a lawyer if you have a question about a legal dispute.