While the term "sister" may have been popular in the 30's (although I don't recall any old time movie stars saying anything like 'Not this ship, sister!") it was still very much a product of the 70's. I think it was just one of those things that Ford slipped in to make the dialogue sound more "natural," although he miscalculated by using lingo that would definitely date the film. As for 3P0's AOTC humor, it still gets a smile out of me and I really don't see what the big deal is. To my ears, it's no worse than 3P0 nattering on during ESB's carbon freezing and subsequent escape scene.