That's right. Although it would probably be more accurate to say Breton is a Brythonic (P-Celtic) language as it is still used today. Yes, it is very close to Cornish. The Ancient Gaulish language was in the P-Celtic family as well. The relationship between Cornish and Breton is not dissimilar to that between Irish and Scottish Gaelic.

The eastern portion of Brittany speaks Gallo, which is a Romance language. It, too, is under threat from French language dominance. Since it belongs to the same family it is easier to absorb and assimilate. I wonder how many Québécois are descended from Bretons, given that the Bretons were well-known explorers and sailors?