The subject of this article originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
Idis was the daughter of Theoden and Elfhild in an early draft of Lord of the Rings but was soon forgotten and removed once Theoden adopted Eomer and Eowyn. She had no other mention in any of Tolkien's writings.
She is non-canonical as she was scrapped from the books.