Hi. Your daughter is absolutely fine. Her weight is at 15th centile and that is acceptable. Normally it should be at 50th centile but still she is fine.
Genetics also determine weight and height of child. If parents are lean and thin then also babies tend to remain at a lower side.
Next Steps
you can give her calorie dense food which includes fat and carbohydrate. add ghee in her food. atleast 1 spoon everyday in every meal. give her healthy snacks like nuts and salads. add soups in her daily menu which acts as appetisers. reduce the quantity of milk.