I need to get temporary custody of a niece

It is unclear what orders are in place, or where they were entered.  If the order was entered in CA, and a parent and the child still lives there, any court action relating to custody would have to be filed in CA.  The jurisdictional explanation can be reviewed in "The Basics of Family Law Litigation," posted on our Published Works page, here.  As a general matter, the available options would appear to be for the other parent to seek a change of custody, or for a third party (i.e., you) to seek guardianship, or even to adopt the child.  First figure out what state you have to file in and then confer with a family law specialist in that place to go over your options.