Is there any way to tell to which FeedItem an EmailMessage belongs? The ParentId field on EmailMessage shows to which Case an EmailMessage belongs, but I can't find any way to figure out the parent FeedItem of the email message via Apex code or SOQL.

The linkage must exist somewhere because the Case feed knows where to display a given email message. Maybe the connection is hidden in an object that isn't exposed to Apex developers?

A similar question was asked here and never answered - https://developer.salesforce.com/forums#!/feedtype=SINGLE_QUESTION_SEARCH_RESULT&id=9062I000000QvsdQAC