We don't normally consult our children before reproducing.
However, adoption is different.
I agree with what others have said, regarding finding out why she isn't on board.
However, if she stays opposed, from my experience, SWs will be very concerned. Even if you get approved, it may well be another hurdle at matching. Children's SW view older children in the family as a huge risk for disruption. I found they ignored the many benefits of a sibling for LO, and spent ages dwelling on how risky birth children are. Are you prepared to go through approval, hoping she mellows a bit, only to find she doesn't, and you can't get matched? Can you as a family unit weather that? Once an adopted child is home, you can't realise your DD is firm in her opinions, and send them back!
As someone said, adoption is a family business, and your DD is a crucial part of that. Adopted children need so much more than birth children. You're not just asking if she'll have a sibling, which she doesn't want.You're asking if she wants a special needs (even if those needs are 'just' ordinary adoption related needs) sibling.
My DS's adore each other. Proper mutual worship. They make casual acquaintances tear up when they see them together, seriously. But we're very early days, and older DS did want a sibling, and very much wanted to adopt.
In fact, older DS has declared I should adopt again, now.
I don't think my liver or waistline could take it.
Adopting a sibling is very very different to having your parents reproduce. And it's going to go tits up if she doesn't come round! So, gently, try and explore what the concern is, and spin siblings in a positive light, and see what happens?