If my 9-month old can sit but not crawl or walk, does she have cerebral palsy?

No. Many babies don't crawl on all fours but rather scoot or roll around. If your baby is doing these things then she is developmentally on tract. If she is not, then she needs further evaluation by your pediatrician, things could still be normal.
No. Crawling may start after walking, walking is normal at 12 to 15 months, and a child with CP would probably not sit unsupported at 9 months of age.
No. Not necessarily. It is important to discuss these developmental milestones with your pediatrician. Many children reach their developmental stages at different paces than the "average" baby and are perfectly normal. Also, some children have delayed motor skills due to other conditions (such as slightly weak muscles) that may get better with either time or special exercises (physical therapy).
No. One cannot say a child has cerebral palsy just because he is not crawling at 9 months. Cerebral palsy is a major neurological problem, and will have other symptoms. At the regular 9-month check-up, the doctor can assess the developmental status as well as examine the baby's movements and muscle tone.