Journey to the River Sea by Eva Ibbotson

Maia, an orphan, cannot wait to reach her distant relatives a thousand miles up the Amazon. She imagines a loving family who she can share great adventures and memories with. Instead she finds two spiteful cousins, who think the jungle is their enemy and refuse to ever go outside.
But the wonders of the Amazon jungle make up for the horrible Carters. When Maia meets a mysterious boy living by the river shores, she begins a journey to the heart of a spectacular new world.

I recommend this book for girls aged 9+, who like adventure books.