My son (now 15) has always had trouble gaining weight. He prefers veggies and fruit to high calorie foods. He never liked Scandi because of the texture. In fact he doesn't even enjoy ice cream or milk shakes- the easiest thing for gaining.
We choose higher calorie food whenever possible (like bread with 120 cal per slice). We add cream and chocolate to milk, that kind of stuff. We found a product called Benecalorie that is a little tub of stuff with 330 calories. You can mix it in other food, but again, if your neice is sensitive to textures it might not work.
It is a constant struggle for us too. People say to me, "just tell him to eat" Yah Right <img src="i/expressions/face-icon-small-smile.gif" border="0"> My kids got g-tubes which took some of the pressure off. But boy, I understand your frustration.