Do you think these two words are antonyms? In fact, they are synonyms, both meaning capable of burning. People get misled by the “in—” suffix: they might think of words such as “invulnerable,” “independent,” or “incapable,” in which that same suffix does make the root word a negative.
However, in the case of “flammable” and “inflammable,” both mean capable of catching fire. Because clothing and upholstery labels still sometimes say the item is “inflammable,” people might assume their couch or sweater will not catch fire. For safety reasons, “flammable” is the preferred usage.