Toddler with no appetite

My 2 year old toddler girl has had no appetite for a year now.  She used to eat very healthy. Since she turned 1 year old she has no interest in food. She does not want to stay at the dinner table, she takes food puts it to her mouth and then puts it down, she will drink milk and water. She is very underweight. She is being tested by her GI doc and they have not found anything as of yet. She is on a feeding tube at night for nutrition.  Her older sister (3.5 years) is a regular eater.  She is small for her age and weighs 20 lbs (she gained weight from nightime feedings - 3lbs :0)).  Please help.