How can I find information on adopted children or children placed in foster care from a cherokee indian tribe?

Hello, I have an unusual background which has made it almost impossible to find any real information about my heritage.  My birth mom was placed in foster care, her mother was a caucasian woman and her father was full blooded Cherokee Indian.  Which would make me one quarter Cherokee but I do not know the name of her father and I am trying to find her birth certificate although I do know her name.  Problem is she has taken on the last name of her step father who I believe adopted her.  She is the oldest of 7 children and she is the only one with different DNA on her father's side than the other children.  This tells me that her mother possibly had an unplanned pregnancy with a man who was Cherokee and once married to the father of the remaining children, he adopted my mom.  

My mother gave birth to me when she was 16 and ended up allowing my paternal grandparents to adopt me.  As you can see, there are so many obstacles to try and get through to find any history of heritage.  I desperately need help and hope someone has some tips.