Addon:RelatedRelativesGramplet
The Related Relatives Gramplet shows all persons in your database that have or had a relationship with a relative. For each found relationship with a relative the relationship is named and the common ancestors for the relationship are listed. The persons in the relationship and the common ancestors are shown as links. You can select either by a single click.
Hint: If you select a common ancestor for a relationship with a relative and create a hourglass report for this person you often can easily spot the related relatives by looking for lines that are crossing the other lines.
You can download this plugin by following the directions at Third-party Plugins.