How do I get my cat to gain weight?

Check her food and see if it has wheat or gluten in it and try her without those ingredients. It could be an allergy to wheat, I might be wrong but it might be the reason. Maybe try just meat and see if there is an improvement. It may to be an allergic reaction. Also cats can have irritable bowel syndrome which gets worse with stress and rich foods (high calories) and it can be similar to her problem. If you change her food it can take a while to see a change. Your parents and vet are doing all the right things.