You've received excellent advice from the posters above. We've adopted many times (8)....and three of these were older adopted children. One had severe RAD, while the other two had attachment disorders of a lesser type.
Still, all three have failed. The most severe went to residential housing after living with us for four years. He is an adult now, and hasn't healed one step further from his RAD issues, than he did when he left our home at 11yrs.
RAD is extremely difficult to deal with. It makes your home and life---hell. Other than having children with sexual predatory behaviors in your home, I can think of no other behavior that is more dangerous---or more difficult to deal with.
HOWEVER.......this child IS very young. There IS hope, because of her age. As the others have said, this doesn't mean healing will happen. The incidence of children with RAD who DO heal, isn't high. HOWEVER, it is a fact that the younger the child is when treatment begins, the higher the chances of healing.
That said, IMO, I would definately delay the finalization of this adoption for at least several months. (It's not as if the child is going to someone else, correct?) And, as the others have stated, I'd find an attachment therapist ASAP. ONLY an attachment therapist will be able to help this child, and give you and your family the proper tools to help this child heal---if it's to be done at all.
Your last post said that you and your dh are willing to work with this child. However, you also said you wish you'd never been contacted by your cousin to take this child.....IF you choose to discontinue your work with this child......you've not failed. RAD is EXTREMELY difficult to deal with, and I can't stress the difficulty it really is.
You may find this particular forum very helpful to you as well. For me, it was a lifesaver during our years with RAD/attachment disorder. This board has a huge amount of resources (as well as a message forum and chatroom) for parents and caretakers of RAD children. It is the Attachment Disordered Support Group:
Attachment Disorder Support Group
My best to you in whatever you decide.
Most Sincerely,
Linny