What is the ruling of Islamic law regarding the determination of the length of a woman's hair, and is it true that it is not permissible for it to exceed the last bone in the spinal column?

What Allah and His Messenger have made lawful is lawful, and what Allah and His Messenger have made unlawful is unlawful; and what He has remained silent about is pardoned. The statement made by your brother's wife that lengthening a woman's hair is unlawful or disliked is speaking about the religion of Allah without knowledge, and this is one of the gravest major sins, as Allah Almighty says: "Say, 'My Lord has only forbidden immoralities – what is apparent of them and what is concealed – and sin, and oppression without right, and that you associate with Allah that for which He has not sent down authority, and that you say about Allah that which you do not know.'" Ibn al-Qayyim said that speaking about Allah without knowledge is more forbidden than polytheism (shirk (definition)). There is no evidence that specifies the length of a woman's hair, and lengthening it adds to her beauty, so there is no harm in her doing so.