Hmm. It's tricky, really... I've experienced the family "know it all" too in case of "I know better than you even though you live in your body" kind of thing. I was diagnosed with depression and took medication for some time, in conjunction with therapy, to help straighten things out. I was told repeatedly "you don't have depression, you're just attention seeking." I promptly dropped all discussion of my medical history. When other person attempted to bring it up, I told them calmly, "That's between my doctor and me. I'm not going to discuss private information with you." And then physically turned away. (This worked in my case because I am deaf, and turning away severed all attempts at communicating with me.)
I will say that there are times when you WANT people to question diagnoses and medications if they think they are not working for you or working inadequately. I remember a case of a friend's father who was being treated for Disease Y when it ended up that he needed to be treated for something else, something that another friend recognized through their own experiences. The family friend sat down with the father and explained what they thought was happening, and politely urged the father to at least bring it up in discussion with his doctor to double check that they didn't miss something when they were diagnosing. The side effects of Medication A were not pleasant, and changing to Medication B helped his condition and improved his life. BUT it's all in how you execute it. A polite discussion of concern ONCE is all that's really permissible. Beyond that it gets aggravating and becomes nosy and so forth, as the OP knows.
In the case of the book, quietly dispose of it. Do not mention the book to your sister. If she brings it up, and she likely will, you might say, "Oh, I glanced at it, but it's really not something that will be helpful to me as I do not have bipolar disorder. I gave it to someone who might find it useful." If she starts in on her usual lecture of 'CRUD MONKEYS! you have bipolar stop denying it!' Tell her point blank, "My doctors and I have agreed that I do NOT fit the criteria for bipolar, no matter what you believe. You are doing more harm than good by continuing to harp on this subject. I feel hurt that you feel that your need to be right is more important than your love for me. I am asking you to love and respect me enough to let this drop and stop discussing this from here forward." She'll probably continue whining to your mother, who will complain to you, at which point you can tell your mother pretty much the same thing.