Hi and welcome to the Chiari forum.
First let me thank you on your sister's behalf....not many have advocates to help us, so for reaching out to learn how to help her is remarkable.
Next, you do not have to refer to it as Arnold Chiari....Chiari is fine.
There is an organization in the UK called the Anne Conroy Trust, they have Drs that specialize in Chiari....that said, research the Drs...just because they treat Chiari does not mean they may be the right Dr for your sister....so do research and educate yourself on what questions to ask the Drs and that is the key to finding that right Dr.
Many with Chiari also tend to have Hashimoto's thyroiditis. so make sure she has an ultra sound of her thyroid and her free T3 and free T4 and TPO antibodies are looked at not just her TSH.
May I ask a question, since she was very active prior to her Chiari symptoms surfaced so that she had to take notice, what was going on in her life at that time....any falls, accidents, etc?
Since Chiari is congenital, many of us have symptoms ALL our lives but tend to not realize that we have symptoms until they worsen....they we take notice something is going on.....but we may have always had issues with swinging on swings, or laughing, talking...all sorts of things can cause issues but since we always had them we consider them and how we feel to be "normal" and we are able to do "normal" activities....until one day we can't.....
I had my PFD surgery in May of '09....it took years before I realized all my benefits from the surgery....am I the same as before....NO...do I still have symptoms...YES ! Surgery is not a cure or a fix, it is meant to restore CSF flow IF there is an obstruction...and surgery will not hep if there isn't an obstruction.
It is important to add that I would not be able to do what I am doing now had I not had surgery.....but, I still have limitations. I no longer drive, which was the hardest and continues to be a difficult thing for me....it is a loss of independence and just when I think I am going to try and drive my vision gets all wonky and I realize it's for the best that I not drive.
What testing has your sister had? I am aware she must have had at least a cervical spine MRI....but has she had any others? Thoracic, lumbar spine MRI and a CINE MRI?
Were any other co-morbid conditions ruled out?