The Sister of Ursula

Is this a sequel of The Little Mermaid, where Ursula, the Sea Witch's sister decides to take revenge?

Or is it a Friends reference? On Friends, isn't Phoebe the Sister of Ursula? Why don't they just call it Phoebe? And why do they use "of Ursula" when a simple apostrophe S will work. Ursula's Sister has a better ring to it.