Do I need to get a lawyer or to court in order to get temporary custody of my niece? Both parents are in agreement in giving me temporary custody until they are stable and capable of taking her back. Is the 'Temporary Guardianship Agreement' form all I need? If both parties are in agreement, should it be fairly easy?

If all parties are in agreement, signing a Temporary Guardianship (get it notarized) should be sufficient to allow you make decisions for your niece. Guardianship and custody are different items but very similar.