thats great that you are so involved in your little sister's well being! If a child is getting very poor grades that appear to be well below her ability level, I agree with you that figuring out why this is happening is very important. You may need to go through an initial visit with your pediatrician to learn more, as your primary care provider may be the referral you need to have your insurance pay for psychological testing (if you are lucky enough to have a company that will cover any part of it). Its always a good idea to rule out anything like a hearing loss, vision issue or other medical problems that could be interfering with her functioning.
You can read my Medhelp articles on ADHD as well as the one on getting an assessment. Schools do not generally diagnose ADHD, and unfortunately lots of physicians do so with a checklist or two. I think checklists are fine as a supplement to a thorough study of the child's cognitive functioning and achievement, because symptoms of ADHD can occur for other reasons besides ADHD. Having a psychologist do a thorough assessment and consult with the pediatrician will give you the information you need to intervene effectively.