Compare https://musicbrainz.org/ws/2/release-group/2d340d43-a128-42c9-8bba-c92f26fc41be?inc=artist-rels+url-rels and https://musicbrainz.org/ws/2/release-group/2d340d43-a128-42c9-8bba-c92f26fc41be?fmt=json&inc=artist-rels+url-rels

It seems the only way to figure out what entity a relationship is a link to is to loop over a list of entities looking for relations[0]["artist"], relations[0]["release-group"], ..., relations[0]["url"] until you find something. The XML WS on the other hand says target-type="artist" and target-type="url".