Alex & Emma

Hey, guys! :D I am Ann and I'm 13 and I have a huge problem.
I can't really remember the name of my childhood favorit movie. -.- :D cause it was a long time ago! I know the name of it in my language and all the scenes from it. Pls help! Here are some hints:

Help me name a movie.
It's about 2 sisters whose mum died. Their father is teaching in a school. One night, a troll (or some mysterious creature) comes to their house and lead them into a ship. I think that there's a mermaid on the deck. And, there, the older sister gets the prettiest stuff and the younger one looks like a peasant. And,...