More important than the parent? Utter bollocks!
yes, she is blood, the OP is adopted, the child is 12.
the grandmother has been in his life since birth, and is still relied on constantly for child care.
As I said, she is POSSIBLY more important to him, we can't know whether she is or she isn't, but she might be, and if OH died, she really could challenge for custody.
I think its lovely that the OP has been so loving and caring towards him throughout his life.
But as a fosterer and adopter myself, I am well aware that the child has no say, and you have to be prepared for the love to be one way.
The OP choosing to adopt the child doesn't give her rights over him above and beyond his grandmother.
adoption was what SHE wanted, and its great she did, but HE owes her nothing