From your measurements, her weight is just below the 5th percentile and her height is 15th percentile for her age. Blood-work will show if she has a deficiency of any type, however, if you are comparing your daughter to your niece there are different genetics in play here due to two different families. Plus your daughter did have issues as an infant that may have set her back a bit. As long as she has been steadily increasing her weight and height regardless of how slow it has been, she will be fine.
She just turned 8 in June
Would you please state the age of your daughter so I can answer your question properly. Thank you.