I need help...?

So here's the deal. My sister is in Seattle for a bone marrow transplant. She is divorced and has a two year old. Since she has been gone, her ex takes care of her daughter during the week (taking her to daycare and babysitters), and I take care of her on weekends. Yesterday, her ex husbands mother brought my niece over, said her dad took off, and that she didn't want to take care of her. My sister can not come home for another three months. I am only 20. I work full-time during the week. I love my niece to death and have no problem taking care of her, but is the anything I can do to get help? Any help, whether help with taking care of her while I work to help financially. I'm just not sure where to turn to get this help, seeing as I am not the mother, and have no way of getting in thouch with my niece's dad. Any help would be appreciated.