In this paper, we consider the grammatical inference problem of slender context-free languages from the point of view of cryptosystems. We show that the inference problem of slender context-free languages is not hard, and therefore, the languages have some weakness as cryptosystems. Then, we propose a hierarchical construction of families of slender languages by using some type of linear grammars and slender context-free languages. This makes the grammatical inference problem of those families harder, and therefore, the cryptosystems using our method become safer.