After learning her mermaid mother has only a week to live, a disfigured teenager must choose between a conniving snake and a vengeful clown, the only ones who know where to find her.

How about something like this?

Rescued from a life of bullying and abandonment, a young girl with a great destiny proves her worth while searching for her estranged mother on a fantastical island located at the end of a rainbow.

How about something like this?

Rescued from a life of bullying and abandonment, a young girl with a great destiny proves her worth while searching for her estranged mother on a fantastical island located at the end of a rainbow.

I think you are trying to say the lead character only has a week to find her mom who is dying and there are only two possible guides who can find the leads mother, both of whom which are flawed.

However if that is the case, then the inciting incident would not be the cancer but would be the discovery that her mom is missing.