I'm wondering if they are just nicknames given to the type of enemy, or if they are really "big sisters" (in oppose to "little sister") and men inside the "big daddy" suit. I know it really doesn't matter, but I'm very curious about who these enemies are.
Sisters and daddies are indeed people, despite their monstrous appearance. However, they aren't necessarily related by blood or otherwise.
Little sisters are little girls who have been implanted with a parasite that uses blood to produce Adam. Little sisters collect and drink the blood of corpses to feed those parasites.
Big daddies are men, who have been physically and mentally altered. They were created as a countermeasure to people hunting little sisters for the Adam they produce. Delta, the protagonist of Bioshock 2, is one of the first big daddies in existence (possibly the fourth, as "delta" is the fourth letter of the greek alphabet).
The Big sisters are merely little sisters after they've grown up.