Does anyone know what its like to have custody of a family members child?

I know I have written about my family and all that has been going on with fighting the state. I have temporary custody of my niece and she is truly a precious child, but it is getting harder everyday knowing I will have to give her back. Even though I knew that this would happen, it doesn't change the fact that I will be heart broken. I tried not to let myself get so attached, but that is impossible when you have this sweat little girl in your home everyday, you cuddle every morning and every night. She tells me she loves me so much and she misses me when she is gone. It's really hard, raising a child that is not your own and that has to go back to its own home. Its also very confusing for her. She has been with me over seven months now and she is comfortable here, but also she yearns for her mommy. I have done the best I know how to make her feel at home and secure, but I am not her mommy. She is extremely jealous that her sister gets to be home and she doesn't and that is hard to explain to a four year old, why she can't be home when her sister is. She is so sweat to my daughter and waits on her every whim. Whenever she is at visitation my daughter asks nonstop when her baby is coming back and one of these day I am going to have to tell her that she is not coming back. But at least we are family and will be able to see her, its not like she will disappear out of our lives for good. I have been taking a lot of pictures lately, I thought if I made my daughter her own photo album, she could look at it whenever she misses her baby. Its just really sad. I am so stupid for letting myself get so attached, but this is an impossible situation. My husband and I have been talking about whether we she remove ourselves from this situation, but we can't that would hurt her too much and we can't do that either. What are we going to do?