This is on mobile so disregard my bad formatting and grammar.

So my younger sister wants to go on a school trip but it’s going to be in Puerto Rico and we live in the US specifically NC. My father is against her going because he sees no big reason to go that far away for a school trip. My mother simply doesn’t feel safe about her going to another country since my mother cannot chaperone. My sister’s argument(she is 13) says that she wants to go because she probably won’t  have another chance to go. I really did not want to get involved but they essentially had me as a tiebreaker on wether she goes or not.

I chose for her to stay. My reasoning is that she is still 13 and will still have lots of chances to go places and visit other countries, but she doesn’t know that I’m against her decision yet.
(I was asked by my parents after she went to sleep) Now I have the task of giving the news to her that essentially from her point of view will make her seem like her family is against her. I told my parents that I will explain to her that she cannot go without really thinking how.

So how should I tell her the news in the nicest way?