After 30 years of struggling with all sorts of issues, some similar to yours, lots of Antibiotics and terrible anxiety and trying ALL sorts of different herbs, diets and meds I've finally learned the secret - you have to RESET your gut flora. I'm no doctor, but my best advise is to research GAPS - it stands for Gut and Psychology Syndrome - look up Dr. Natasha Campbell-McBride, she's a neurologist and Dr. of Nutrition. I heard her speak at a Westin Price Conference (look them up too) her program is changing my life. It's mostly about an Elimination Diet and very slowly adding foods back in. This would also apply to your brothers autism.
What she discusses about stomach health and illness is astounding. YOU can do this diet - you drink a LOT of broth. For me, anxiety gone, headaches gone, knee pain gone, shoulder pain gone. You don't have to buy the book you can learn all about it on her website or via a small book called GAPS Guide. Oh, your question about heavy metals and how we get too much in our system, vaccinations, metal fillings, many meds as they have preservatives that contain mercury. This diet will address that issue as well as your candida and possibly parasite issues.
You are so fortunate to be working on this now, being as young as you are. Take your time and learn all you can before you do anything else.