Try and invovle her as much as you can, like consulting her on names (pick a list and let her help you decide etc), what she'd like in the baby's room etc and keep her updated on how you are going. Despite everything, it sounds like you have a really good relationship with her and I think she may even be excited about being a big sister. When you tell her, maybe you could say she is one of the first people to know and it will make her feel important being part of a 'adult' secret IYKWIM? I have heard of the baby 'giving' a gift to older siblings too, which would be a nice welcoming gesture. And like Rayla said, all you can do is counter any negativity with love and understanding.

I hope it all goes well for you.