https://musicbrainz.org/ws/2/release-group/315f9309-b69c-4c48-83df-273e2d7a4422?fmt=json&inc=release-group-rels helpfully includes "relations":[

{"direction":"forward","type":"single from"}

]

https://musicbrainz.org/ws/2/release-group/315f9309-b69c-4c48-83df-273e2d7a4422?inc=release-group-rels includes the release group as expected.