It’s certainly possible that boosting her calories (particularly carbohydrates) would improve your sister’s digestion and help her feel better. Unfortunately, it’s very unlikely you’re going to be able to convince he to try it, unless she’s the one who came to you for advice. In my own experience, I’ve learned that it’s better not to give dietary advice unless the other person expresses an interest. It can be hard when you care about a person, but she probably doesn’t want to hear it right now. If the problem continues for several more years, she might change her mind.
I’m currently experimenting with an approach to increase calorie intake without gaining weight. You can check out my thread, “A complicated recovery,” to read about it–though again, I doubt your sister will listen unless she’s reached a point where she’s open to advice.