*Giant Hug* I know how you feel. My whole life my family has thought I was "Faking it" since they were all healthy. I was told from when I was a child on that I was a burdon and an imposition. Even when I was sick with more then just a migraine, such as when I got gallstones in 8th grade and they had to remove my gallbladder, my father's and brother's attitude (which my father also passed to his side of the family, my grandmother, my uncle, aunt and cousin) was that I was some how faking the tests and tricking the doctors. He even asked the doctors in front of me how I would have faked the test and told them to ignore me that I was lying. When the doctors looked at him like he was insane & told him I wasn't faking, because there is obviously NO WAY to fake an MRI, a CT, or an Ultra sound, he still didn't believe them. He thought/thinks everything I suffered through I did deliberately to upset or anger him and his favorite pet name for me to this day is "You lying, petty spiteful, ungrateful, little b*tch". Sometimes he changes it up and substitutes "Sh*t" for the B-word. You know you've not had a full life til your brother hears your father telling you to "knock off the lying and just go kill yourself already" so that you'd be less of a problem to the family and then begins repeating that himself when ever he's wanting to hurt you. My brother is 3 years younger then me and spoiled rotten by my parents because he's the boy, as well as being a very materialistic, ego-centric person. So as you can guess, he said that sorta thing alot.
Its also "fun" when my only remaining grandparent rolls her eyes and tells me in a very angry voice to "knock it off" when I try to excuse myself to go laydown when we're up visiting her and my pain gets particularly bad. Because of my father telling her I'm only faking and her being 90 years old and looking like a 60 y/o that's as fit as a 40 year old, she assumes I'm lying just because I've had more illnesses, operations, and medical issues at 30 years old then she has in her whole life. That's a very hurtful moment as well, to realize your Grandmother thinks you're a lying hypocondriac who tries to make up things about your health to cause problems in the family. I still meet people and make friends with them, only to have them begin to assume later that I'm a hypocondriac when I have to cancel plans when I'm not up to it. They all have the idea that if you're physicaly sick or hurt, they should be able to see it with the naked eye. People that like I explain to them once what's wrong with the medical terms and information and if they still think they should "be able to see it" then they're just too stupid to understand and are just not worth my time.
So, I completely feel your pain and truly sympathize. I know there's no real way to help with it. Its not okay that they do that. Its never going to be okay that they act that way. Its because they are ignorant and selfish people that can't extend their cares to inculde people other then themselves. The only thing I can say is that your brother in law must be a very self-involved person and he just needs to grow up and learn to not talk. He might be so childish as to be jealous of that fact that your sister loves you enough to help you with things and to spend so much time with you. But imaturity in him is no excuse for his behavorior, nor is it your fault that he's an idiot. the only thing you can do (assuming you've told him in the past that he needs to grow up and learn not to spout off about things that are far beyond his small minded educational grasp) is to ignore him. If he speaks, look through him. He's simply not worth your time- I truly mean this, he's not worth a moment of the time you have on this earth. He's a small person that does not matter. If he makes your sister happy (which I'm doubting if this is the way he treats people she loves) then good for her, she must be a loving, giving person that deal well with even the meanist, most unworthy creature, but he's not worth the space he takes up in the world if that's the way he treats others.
Now as for the children... The 16 year old should very well know better by this time. Your choices for dealing with her are a couple of ways. If you think she's just ignorant and doesn't realize what she said truly hurts you, then tell her plainly that what she said hurt you and you'd appreciate if she wouldn't repeat lies that others who don't have the education to understand spout off. Tell her if she has questions about a medical condition that she doesn't know about yet then you'd be happy to allow her to come with you to the next appointment to ask the doctor. If she said it to be mean or hurtful then you're welcome to remind her that she's currently living with you, out of the kindness of your heart, to help her deal with her own problems. If she's moved out of her parents house to do this it implies that she's got issues w/ her parents which you might want to remind her that if she already disagrees with her father over things then she should already realize by now that he speak about things that he doesn't know about and she should know better then to repeat things that she knows are wrong. (I'd like to put it more bluntly and tell her to stop repeating her father when she knows he's talking out his azz, but you might want to put it the polite way.)
As for the 8 year old, you'll just have to sit down and have a talk with him. Print out some Patient information stuff that's simple for a kid to understand, or picture for him to see, and just sit him down and tell him "Honey, your father doesn't like to admit I'm sick because it scares him. What he says when he's scared hurts my feelings, so please don't repeat your father. This is what's wrong with me: I have something called Chiari Malformation. This means that my spine pushes on my brain. It hurts alot and causes other problems too. But you don't have to be scared for me because the doctors are doing their best to help me it just takes them time. I love you very much and I wanted to tell you what's wrong with my body so you would know and not worry." Kids ussualy just repeat things because they don't know better. Once you set them straight it should pass. If the child repeats what you said to the brother in law, that's good. Maybe he'll learn to shut up, if not and tries to tell the kid you lied or something just get him and your sister together and tell them what he said hurts your feelings and that he's also hurting your relationship with the kids when they hear him say thing like that and then repeat it. If he's not a complete jerk he should stop.
I hope some of that helps, but it will still be hard for you to try to talk to them about. Just be strong, and simplify for the kids so the big words don't scare them, and with time it should work out. *Huge Hugs* But just remember, you're not alone!