Hello, D.~
God bless you and your husband for what you are doing!
You have received some very good advice here. Let me add just one thing: as soon as your granddaughter is old enough, get her some professional counseling, preferably with a Christian. This is why I say this- My own Mom was raised from birth to 13 years old by her grandparents. Her parents had lived with his parents, and when they were expecting the next child, moved into their own place and left her there. When her Grandma died, Mom had to go live with her parents and siblings; back in those days it was believed a man should not raise a child, especially a girl. Grandpa immediately found a live-in babysitting job for her to do. Mom just turned 85. It still bothers her that she never knew why "Mom and Dad gave me away to Grandma and Grandpa. Why didn't they love me, too?" She was the oldest of a very large family. She actually had a much better life than them, except for being raised like an only child and lonely. She had clothes on her back and plenty of food and got to go to school every day. Her dad was an alcoholic until I was around 12, when he went for "the cure," and literally could not stomach the smell of aftershave w/alcohol base anymore. It seemed to work; I don't know why it isn't done anymore. Anyway, my dear Grandma raised a huge house full of kids with absolutely no help except the other kids, nothing from Grandpa but money if he had it. You can imagine it was not easy. When I was little and Great-Grandpa could no longer stay alone, he came and lived with us. Mom was so happy to have him there!
It's extra hard because there is another child they kept. It feels like you weren't good enough, but the other(s) was/were.
Anyway, all that to say this: without good help to deal with this early in life, it can be painful for nearly a century. I know you don't want that for your little angel.
Thank you for loving your granddaughter.
Blessings,
S.