On the verge of a succession crisis, a good mermaid princess is banished from her kingdom to the human world for her forbidden love of humans; but when her two wicked sisters usurp their father’s throne and destroy each other in a power struggle to rule the kingdom alone, she must choose between humanity and saving her people from the tyranny of her sisters.

Agreed with all the above comments.

Also, and this may be a wording issue, I'm not clear on who gets destroyed. Do the sisters kill the king and then destroy each other? Or do they destroy someone or something else in the process? Who is the MC needing to save her people from?

A decision tends not to be enough. This is huge amount of setup. All good but I don't see the story. Is the banishing in the script or just a few pages?

What is her goal in life? Try it from that angle.

I like dpg's point about; a story is not about a choice but what they do about that choice.

Along those lines,

1: If the lead chooses to go back home to stand up against her sisters, she would be 'doing something' she would be taking action.
2: If the lead character chooses the human world, she would be 'doing nothing'
3: Since lead characters are active (Do something) not passive (Do nothing) as written it is a false choice. (A good lead character isn't going to choose to 'do nothing' over choosing to 'do something')

4: So (As dpg was basically saying) I would drop the 'having to choose' angle in the logline and just start (The logline) with her sisters taking the throne and the lead (As a result of her sisters action) returning home to save the kingdom.