Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
2. sībĭlus, a, um, adj. [1. sibilus], hissing, whistling (poet.; occurring, on account of the metre, only in the form sibila; cf. 1. sibilus init.): colla (colubrae), Verg. G. 3, 421; id. A. 5, 277; cf. ora (anguium), id. ib. 2, 211: coma torvae frontis (Panis), Val. Fl. 3, 50.