Something that is yours forever, that can’t be taken away and given to your little brother instead? That something would be called inalienable. The word refers to a natural right that cannot be revoked by an outside force.
The word inalienable is often linked to human rights — you’ve probably heard the term “inalienable rights.” In the Declaration of Independence Thomas Jefferson (using the un- variant) wrote that all men are "endowed by their Creator with certain unalienable rights" including "life, liberty and the pursuit of happiness.” The spelling may vary but the meaning is clear: an inalienable right is something that can’t be given or taken away by a government or another legal power.