I'm not sure if this will be helpful, but it's the only thing I can think of. I recently joined a Facebook group called The Gymnastics Doc. I don't recall the owner's qualifications, but he's there to answer medical questions from gymnasts/parents. I remember he did a video - I think on shin splints - recently and there was a part about the knees. Something like a way you can work to make them straighter. He said it was a common problem - I think it can be due to tight ligaments or growth. You might look into it. I asked my girl about that video when I watched it, and she said that her coach has the girls sit on the floor with their legs out in front of them and another gymnast gently sits on her knees to stretch them. I'm NOT RECOMMENDING this technique!! I'm not a coach or a doctor. I'm saying that it seems to me from these two bits of info that 1) this is something you should be able to find information on and 2) that there must be stretches/work out there to help. If I were you, I'd join that Facebook group, check out the videos, and maybe post on the forum (if you can) to ask him specifically about your situation.
It's the best suggestion I have. Good luck!!
