Mitochondrial DNA analysis shows that G. capito and G. sororum are more closely related than either is to the other Geodorcus species found on mainland New Zealand.

[5] Other Geodorcus species are associated with rotten logs, but this habitat is absent from the treeless Sisters Islands, which are two pyramidical rocks about 100 feet high, covered with scanty bushes, and frequented by countless numbers of sea-birds.

Geodorcus sororum larvae must consume an alternative, as the plant communities on Middle Sister Island consist only of small shrubs and herbs.

The adult beetles must also consume an alternative food source to the other members of this genus, considering the absence of woody vegetation on this small island.

[4] Like all Geodorcus species, G. sororum is absolutely protected under Schedule 7 of The 1953 Wildlife Act, making it an offense to collect or harm a specimen.