Congratulations on your DD.
I just wanted to say, please don't feel your DD is "testing" you by rejecting you for your mum, she won't know which way is up right now. It could be as indiscriminate as that your mum's face (or item of clothing, or perfume, or voice, or hair) is more reminiscent of her FC, or someone else who she has felt safe or happy with in the past. I don't automatically think "testing" as such. Obviously I don't know her however.
I also wanted to echo Faeries comment upthread - "I have experienced this from all sides. Please don't deny them of their memories n past or underestimate their bonds. It will be hard work but they will grow to love you and accept you as their parents". I too have experienced this from the point of adoptive child. I felt that my adoptive mother struggled to get over the fact that our birth mother, and extended family (although estranged) had a connection to us that she didn't and couldn't ever have (that of blood ties).
My sister and I were young (I was 5 1/2) when we were adopted but I still remember having to call adoptive mother "Mummy" and it was puzzling because I already had someone I knew as "Mummy" (birth mother) and also I didn't know how to explain to school friends, neighbours, shop people, teachers, Brownies, etc that I had two Mummy's and which Mummy I meant when I said Mummy. My birth mother had a red coat so for example I might see someone wearing a red coat and tell them "My Mummy has a red coat like that, I mean, Mummy who I used to live with not Mummy who I live with now". Or I might tell a school friend "Our cat is called Snowy, that's the cat at Mummy who I used to live with not Mummy who I live with now, she has a dog called Ben". It's not intentional to talk about birth mother but connected details can just slip out.
I also knew when my mum (adoptive) was deliberately hedging on things like photos and letters and details as I got older, ("Maybe later", "I don't know where they are right now", "We'll have a look after the weekend" etc) and I found it annoying. Now I am a mum myself, I can clearly see both sides of the story, that she loved us like we were her own blood daughters, which was lovely, but that photos and letters of our birth family were an unwelcome reminder that at the same time there was and is no getting past the fact she was not our birth mother.
Again I don't know what if any is helpful from this but I guess I just wanted to add that Faeries comment touched my heart because that's how I feel too. I know each case is individual but I just wanted to...throw my experience in from the other side, as it were. That birth mothers and families are often seen differently from the point of view of the child than they are to everyone else.
All the best to you and DD, you sound like a marvellous & caring parent xx