Yes, an AC is more likely to hurt you BC than an second birth child would. The AC will bring all their trauma, and the drama with that, into your home, and it will affect your BC for the rest of their lives.
But, so would having a BC with special needs.
Adopted children all have 'special needs', they all, in some way, have suffered huge loss, and that affects them deeply. Many will have been exposed to drugs or alcohol, or both, and that makes them special, and needy. By adopting, you're choosing this for your family, this difficult path. With a birth child, you're throwing the dice.
I think a lot depends on whether a birth child is possible. Whether you have really really given up on that plan, as you can't apply to adopt and still be trying.
If adopting a sibling is your child's only chance for a sibling, and you want another child, and you want that child through adoption... well, then it can be a wonderful and beautiful thing!
My BC and AC have an amazing bond, and it really is beautiful. But AC does hurt BC, and BC does make sacrifices. BC is also empathetic, compassionate, and is learning many skills and traits that many adults lack. AC is a blessing on our lives.