I'm adopted and have always know it. It was never hidden or kept from me - just knew that I was extra special as I was 'chosen'.
My grandparents on my fathers side did accept me but were unfair when it came to things like presents etc... so possibly not as accepted as I could have been or as my cousins were.

If you grandmother is not accepting of the child, I think the best option is to just make sure that she feels loved by the rest of the family, that she knows that she was 'chosen' and that she is very much wanted.