In diachronic conlanging
The term conlang family usually refers to a language family of diachronic conlangs interrelated through their imaginary history, especially their historical phonologies, tracing them back to a con-proto-language. The creator of a conlang family, the most notable exemplar of which is J. R. R. Tolkien, usually regards the conlang family, embodied in the interrelated histories of the member languages, as a work of art in its own right, of equal or even prime importance compared to the individual member languages.
A conlang family may arise 'spontaneously' in diachronic conlanging through the principle "When in doubt create a dialect", i.e. when the diachronic conlanger is in doubt which of two (or more) alternative paths of historical development to let their histlang take they project several dialects or sister languages, each following one of the proposed paths, or assigning alternative linguistic changes to previously projected dialects or sister languages.
Another, less frequent, usage of conlang family is to refer to the conlangs of an certain conlanger, seen as a family by virtue of being the creations of the same artist.