Due to their external similarity convergently acquired by strong selective pressures due to the identical habitat, they were occasionally believed to be close relatives. However, they belong to two well-distant families in the Sylvioidea, the "warbler-and-babbler" superfamily:
Cettia, the cettiid bush-warblers or typical bush-warblers, belong in the Cettiidae, an ancient sylvioid lineage related to long-tailed tits.[1]
Alström, P.; Ericson, P.G.P.; Olsson, U. & Sundberg, P. (2006): Phylogeny and classification of the avian superfamily Sylvioidea. Mol. Phylogenet. Evol.38(2): 381–397. doi:10.1016/j.ympev.2005.05.015 PMID 16054402 PDF fulltext