I think you might get some mileage from changing your terminology from "snacks" to "meals".
We had lunch at 11:00AM and at 4:00 I asked my aunt for a snack. She gave a tuna sandwich, an apple, and a pear, and said "don't eat too much we have a big dinner ready in a few hours". I am supposed to eat 1500 calories at 3:30, not to mention being very hungry.
You asked for a snack, and what your aunt provided many people would consider a lunch-sized meal. Even knowing "Rosemary needs to eat a lot of calories" may not easily translate into "Rosemary's definition of a 'snack' is 1500 calories - which is close to the daily allotment of calories for some people." So be more specific about your requirements "Aunt, I need to eat meals consisting of ~1500 calories every couple of hours. I can provide my own, if that's easier.*" And then if she offers to provide your extra meals, it would be helpful to give her some sort of an idea of what a meal should consist of in order to hit your calorie target. Because I suspect I'm not alone in having no clue how to provide a healthy 1500 calorie meal.
*I really think it's important to offer this because your condition requires over and above normal hosting expectations.