I would say that you would miss very little if you skipped the first game. It's a good game, but the sequel improves on it in every way possible. The first game is literally performing the exact same five actions over and over again the entire game. The sequel completely throws that out the window. And the story is interesting but the sequel is light years better. And you play a different character in the sequel so the only story you need to know is from the brief framing story which you could get reading the wiki synopsis.
It's like AC1 is the Hobbit and AC2 and ACBro are LOTR 1 and 2.