You really need to lay out for her all the pros and cons of an older child adoption. Since you are already in the position of seeing what comes with an older child adoption, why don't you lay out a typical week in your life and then see if she would be able to do all the things that you are doing - therapies, bonding activites, etc.
In our toddler adoption, things have gone exceedingly well and it has given a false sense of ease to others who are considering an older child adoption (greater than 24 months). I can tell you that out of 5 families in our travel group, we are the ONLY ones who have not had significant problems. Two families are dealing with RAD, another with developmental issues, and two with undisclosed medical problems.
Perhaps you could point her to some good books to read about adoption and the issues that come with it. That way it is another source telling her things and not just you.