It does not sound like anything the wife discovers on her dead husband's Facebook account can result in YOU doing anything, unless you disclosed to your brother something so extremely personal about your life (an affair, a sexual disease, an abortion, whatever) that its disclosure to others would be considered invasive of your privacy.
If the husband owned property that he was trying to keep a secret from his wife, well, he blew it by communicating with you about it through his Facebook account. This secret property could very well belong to her now.
When emails are exchanged, the sender of an email has given control over what is written to the recipient. There is no guarantee that the recipient of the email will not forward the email on to others or disclose its contents to others. Confidential information should not be communicated in an email. When information is published on Facebook, privacy settings alone will not guarantee others will not have access to this information. Confidential information should not be published online.
In order to keep a secret a secret, it should probably not be communicated to anyone at all.