What does the Little Mermaid tell the twins about the way people believed her story ended and what really happened? Chapter 18

The Sea Foam Spirit (Little Mermaid) tells the twins that contrary to popular belief, she did not die, but was turned into a sea foam once she found out she was unable to keep her deal with the sea witch by marrying the prince she fell in love with. She gives a continuation of her tale and says that her sisters traded the sea witch their hair for an enchanted dagger. If she killed the prince, she could return to life as a mermaid, but ultimately she still cared too much for the prince and could not go through with it.