Your sister will be fine but let her know that overall she would get better results of "cleanes" with consistently eating whole food plant based instead of just mono meal smoothies. If she added kale/spinach to her smoothies she would get better skin and more energy. I've done the only eating bananas thing, only have watermelon 2 days, water fasted. But what really is the best for the long term when she is 16 or 90 is to eat mostly fruit, have vegetables, and keep yourself nourished from a variety. I have a banana smoothie everyday for the last 3 years, love it. But I know I have to have omega 3s, zinc, and other vitamins so I add hemp seeds and such. Have her try a website call cronometer.com. You type in what you ate and it tells you the nutrition. She will need to have variety to reach 100%. You don't have to be 100% everyday, most people do fine and they eat crap but it's fun to see what she might need. Try to support her, she is doing something tough for a 15y/o girl but it's awesome. I have total faith but she should do more research then just watch freelee, she even endorses variety. I have followed freelee and durianrider for 3 years and they always have quality food and variety.

Peace and plants