What a worrying and upsetting time for you
You're doing a good job and I think you're right to hold this course.
Weight gains do typically plateau around this time and she's still growing well, which is good news.
If there is an underlying condition, however, then it's imperative that you discover what this is before introducing something else into her diet. It sounds like you're doing all the right things and taking medical advice. I'm sorry your family is not supportive of you. That must be very difficult.