How can I add a views relationship to list the group contents of a group

Use case:
I would like to list contents of a group in a view and then relate to that group (using existing relationships), and then relate to other group contents of that group (of type membership), to finally relate from the membership to the corresponding user and add a contextual filter on that user (to be the current user).

In short, that would allow to list contents from all groups a user belongs to.

The only missing link is to have a views relationship from a group to its group contents.