Merfolk are mammals, they're just human upper bodies grafted onto whale lower bodies. Sirens are bony fish, they have fish like features and most of their bodies are covered in scales, their upper bodies and faces are human shaped. False sirens are cartilaginous fish, they're built the same as sirens but whole body is fishy, and faces are kind of otter/human like.

Sirens and false sirens don't have boobs.

You're a lesser guard, you were just patrolling the grounds of your kingdom when you saw your boss, Sunnera, get told by the prince that he couldn't marry her because he didn't like women and was in a relationship to this feral false siren man. The prince was very kind and gentle about it, and the false siren threatened her not to tell anyone. The prince and the false siren swim away, both riddled with anxiety and discomfort, and you're left alone with the dejected guard. I mean, she's kind of a dick but that was just insane.