How can I get a relation by the endpoints using the "Fetch entity by property" action in rules?
I have managed to fetch relation by endpoint with the help of @marcusx and the patch of @MahmoudMostafa (drupal user names). You can find all in the issue Fetch Relation by Endpoints in rules.
I had also linked your issue to that one.
Resuming the steps here, you need to:

apply the patch



Create a list of any entiy
Put your endpoints in the list
Use the endpoint list to fetch the relation

cited from marcusx comment

(I see you already know this).
Bear in mind that

&quot;fetch relation by endpoints&quot; action in rules, [...] fetches all the relations for the endpoints in the chosen list, even if the list contains only one endpoint.
In this (sub)case, the action fetches all relations pointing to the selected endpoint, whatever the other endpoint.
cited from my comment

And that the resulting fetched_relation is a list (I see you already know this as well).
I believe you only need to apply the patch, which you didn't mention in your question. Is it true?