Like I get what you mean but honestly 'She's supposed to be a siren' kind of comes off as more of an excuse to make a character look not-grotesque and more conventionally pretty than it does a proper justification, especially seeing how Isaac normally handles its monsters.
So it's not like the boss was meant to be 100% mythologically accurate to begin with. Mythologically sirens are bird people and/or aquatic (almost always, to my knowledge), and the Siren boss is neither of those, instead being a demon fought in an occult place. From a quick Wikipedia skim, they debuted in Homer's Odyssey and weren't described at all, so being attractive isn't part of their first appearance. You could have a monster lure people through mind control or such (and I'd imagine that this is how the Siren turns familiars against you, because a bunch of dead floating babies being so attracted to her that they turn against the player character, but only sometimes, from looks alone has a lot of holes in it).
They just lure people, which the boss already does. If you wanna get that technical, sirens aren't supposed to be seductive either.
