What is the portion due when one of two sons acknowledges a sister whose kinship is unestablished?

Primary text

If one son acknowledges an unestablished sister, he pays her one-fifth of what is in his hand. This is because he asserts, 'We are two brothers and one sister; thus, you are entitled to one-fifth of the entire estate, which is one-fifth of what is in my hand and one-fifth of what is in my brother's hand.' Therefore, he pays her one-fifth of what he possesses.