Bartleby, the free (as in beer) ‘Internet publisher’, has available a fascinating graphic depicting the evolution of language (all those stemming from the single Proto-Indo-European language, anyway).
Now I know the historic route of my native tongue (as opposed to my cradle tongue), Welsh: it’s closest ‘relatives’ are Cornish and Breton (in that order) as they are all Brythonic languages that in turn stem from Insular Celtic.
Update: It appears the original has been removed/taken down. I’ve updated the link above to point to a local copy.