B.P.
Neither of my children were walking at 9 months of age. My daughter was 12 months and my son didn't walk until 18 months! (I'm not sure either one was even crawling at 9 mos.) Now both are so fast I can hardly keep up with them! There is an average age for each milestone (sitting, crawling, walking), but every kid is different.
Definitely talk to your doctor about it. If your doctor isn't concerned about her not sitting up yet, then don't worry. But if there is something to be concerned about, your doctor will know what to look for.