The Magic Faraway Tree question

Please help me!! I read a book a few years ago and LOVED it, the only thing is I can’t remember the title! It was about a brother and sister who found a tree and the brother went in then little people who lived in the tree captured him and thought he was a weasel or otter or something. Then the sister went in and befriended somebody of the people and helped rescue the brother. Then both of the children had a spell put on them so they couldn’t tell anyone about the tree. That’s all I remember :(. If anyone knows the title of the book please, please let me know 🙏!!