We have a membership type that is shared by relationship. However, after the membership type was created, we needed to share it by a new relationship type. I edited the membership type directly in the database to add the new relationship type (since the interface does not allow you to edit that after a membership is created). I simply added the new relationship_type_id and relationship_direction in the civicrm_membership_type table for the appropriate row.
It seems to have been working fine, but I am tracking down a bug now and wonder if this edit may be related. (The bug does not seem to happen on the demo site).
Here's the issue: when a relationship is disabled from the Organization or Individual's Relationship tab (ie. using the "more > Disable" command rather than editing the relationship, disabling, and saving) the membership which is shared by relationship is not disabled. When the relationship is rather edited by clicking Edit, unchecking Enabled, and saving, the membership disappears as expected. (In fact, if I do it the first way, then Edit and Save without changing anything, the membership disappears, too.)
UPDATE: I can actually reproduce this on the demo server with a non-Employee relationship so I think it may be a bug. I have submitted a bug ticket on JIRA.