What do we do now?

My Husband and I have custody of my 8 year old Niece. I have had her since she was 5 and she will be 9 next month. My Husband and I were married 1 year ago. Here is my situation. My Sister (the Mom of my Niece) lost all 3 of her children nearly 4 years ago. I took in my Niece as I stated earlier. One of the biggest reasons that my Sister lost her kids was because of her selfishness in caring mostly about herself and putting her needs WELL before the kids' needs. Our county didn't like that and so the kids were removed. Well,........they say that history repeats itself and it has now. My Sister up and packed one night - overnight recently - and left for out of state. In fact, she is about 3000 miles away now and she didn't even say goodbye to any of us, her family nor did she say goodbye to any of her children. None of the children know that she is away. However, it was from the eldest child (age 15) that we found out about 1 month before she left that she had met someone online and was talking about moving to be with him. Again, her needs have gone before those of her kids. My question to you all is this. What are our rights as guardians as we are talking seriously about trying to adopt my Niece. She's been through enough and only bad can possibly come out of a situation such as this one where my Sister is involved. My Husband and I feel that adopting my Niece would give her some closure and make her feel that she has a "normal" home life. We don't have much money but we do both work full time. What are our rights in the state of California and can we seek legal assistance for funding the adoption if we decide to pursue it? I am sorry that this is long but I have a lot to say and this is my first time on this website. Thank goodness there are people like you to come to! God Bless You and I look forward to any help that I may receive.