Epic to milestone mapping

Epics created in JIRA are created as milestones in GH.
Issues created with an epic link in JIRA are assigned the Milestone. 
Issues created in GH with the Milestone are linked to epic in JIRA.

Note that adding an epic link to an issue in JIRA or adding the milestone to an issue in GH after creation has no effect - neither system sends out an event when you do this.