My 18 month old niece

I keep my 18 month old niece and I am concerned that my sister in law should take her to see a dietitian.  I have a 30 month old that is over the charts and I have taken him to see a dietitian and it has worked wonders.  I have always been very cautious on the things that I feed him but never really understood the real outlook on the amount of Calories or the Sugar.  My niece is and has always been very small she weights 18 pounds and is less than 1% on  her chart our Pediatrician has sent her to a GI and has had the sweat test done and everything has came back normal.  She thinks that the doctor is not worried  so she is not worry either but all the Doctor can ever say is YOU NEED TO GAIN SOME WEIGHT.  I love this doctor to death but she was never really concerned that he sould see a dietitian either his Neurologist was the one who wanted him to see one.  What should I do,  should she be seen by a dietitian as well?  It just really concerns me my 8 month old weights more than she does.  and as one of her providers I feel I sould check in to this.
Please help.
Wendi

[Renee Radenberg]

I suggest the child should drink high nutrient dense drinks, such as Pedisure or milkshakes (with milk and ice cream) instead of milk for additional calories and nutrients. This will help her gain weight.