I am so sorry to hear about your sister's diagnosis. Don't beat yourself up because you've turned to food for comfort. That's something anyone of us would have done in similar circumstances. I know things are stressful right now, but eating junk isn't going to help. Here's something I've learned from my own years of yo yo dieting,"if you eat crap, you're going to feel like crap".
When a person is under stress, that weakens their immune system. It breaks down their body. Now is when you must try to do your best to eat right. Your body needs those wholesome foods to stay healthy in times of stress.
And if you do feel like you run to the frig, put your coat on and take a walk. Walking is not only great cardiovascular exercise, it's a great way to reduce stress too. Try drinking a cup of tea. Tea can also help you reduce your stress. No one faults you to turn to food, but there are better things you could do instead of eating.
Be kind to yourself as you would to others.