I am trying to get temporary custody my nephew

Why does your sister want to give you tempory custody? Well first of all she would want to give you guardianship, that is tempory, custody is premanent. The consent of the father is required in both cases to get the custody or guardianship. What the mother can do is give you tempory guardiandship by having a notirized statement(Any Blank Legal form Will work) she would need your name her name the state the child will be residing in, the name of the child date of birth, the time the guardiandship will start and the time it will end, her consent for any medical treatment, and this form will be good for any school or doctor, but this notirized form is not legal and binding with the court, this guardianship can be revoked at anytime by the mother upon giving you a written notice that she is terminating the guardianship and is effective as soon as she delivers the notice to you. She does not need the consent from the father, however if he were to find out he could file for custody if there is no court ordered custody already in effect though the court. Usally in most states this notirized statement of guardianship is good up to 6 or 9 months, and would need to be done again.The father has rights only if he is on the childs birh certificate, or was married to you.