The closest known relative of C. briggsae is C. nigoni (aka, sp. 9). Other leading Caenorhabditis nematodes as experimental models that are related to C. briggsae include C. elegans, C. remanei, and C. brenneri. The following tree is taken from the article Felix et al. 2014.
Hermaphroditic species are shown in red whereas male-female species in blue. Image taken from Felix et al. 2014. A Streamlined System for Species Diagnosis in Caenorhabditis (Nematoda: Rhabditidae) with Name Designations for 15 Distinct Biological Species. PLoS ONE, DOI: 10.1371/journal.pone.0094723