Should an article at the start of a subtitle be capitalized?

Yes, capitalize articles (a, an, the)  at the start of titles and subtitles in English:

The Sisters: The Saga of the Mitford Family

A Sense of Things: The Object Matter of American Literature 

Also capitalize articles at the start of titles and subtitles in languages using the Latin alphabet:

Los pueblos: La Andalucía trágica y otros artículos