Frankly, you're offended. You were mer royalty, a possible heir to the crown if your father bore no sons. So what if you drowned a few guys off the coast of the local town? I mean, you're a siren, it's what you do. All of a sudden you're being declared insane and dropped off some random island. Exiled, and miles away from anywhere you could resonably swim. But you know what? It'll be just fine. You'll seduce local mermen, and have your own kingdom. One ruled by your female heirs, as you were denied the crown in the sea.

ISLAND PARADISE REQUIRED!

Download a small island, like the Isle of the Midnight Sun. That's what this challenge is inspired from, after all.
Create a female sim (or male, I guess, but that makes it way harder.) She needs to be absolutely gorgeous, like a siren from myths. She must have the traits Insane and Evil and Irresistible (if you have UL. If not, Flirty), with whatever else you choose. Name her something related to water. Plop her down on your island, and use any means possible to make her a mermaid. Make sure the world has dive lots and such, with harvestable kelp. That's where her food is coming from.

Try and earn her money, plant lots of watermelon, and check in the diving lots for any mermaid NPCs. When you do find one, get pregnant. DO NOT MOVE IN THE NPC!
Here's where the watermelon comes in handy. Feed her as much as you feel is necessary to get a female child, and take care of your siren during her pregnancy. Try and build her a house (hint: Skills like martial arts and others that bring in money are very helpful.)
When your (hopefully female) child is born, give her the Evil and Irresistible trait. This is an evil kingdom, if only inhabited by the mother and daughters throughout the generations.
You can have more children, but they all (except for any boys) need the Evil and Irresistable trait. These are sirens, after all.
Keep these mothers and daughters alive and thriving, hopefully in a nice home, for five generations, and when the sixth generation is born, you can now storm the lands your first siren was exiled from. Take it back, and declare it yours. You are the Daughters of Poseidon, from Rhodos Isle, and you are strong enough to rule all of mer.

I hope you enjoy!

Well, I've been playing this for a while, and it's turning out to be pretty fun! Pictures under the cut.