My dd has 4 diagnoses for serious medical issues, but none of them are apparent to the casual observer, so she, like you and your daughter, is in that "invisible" illness category. They are debilitating illnesses, and they are real, but they're not something you can see, like paralysis, or cerebral palsy, or a physical deformity, etc.
First off, go to www.butyoudon'tlooksick.com and read The Spoon Theory (up at the top. It is very enlightening and perhaps your husband could share it with his parents.
But most importantly, I would urge you to re-think your original question. Some people are never going to accept the facts, no matter how solid those facts are. One time my dd was having a particularly bad day, and a neighbor kid came to the door asking if she could play video games with him. My dd was lying on the couch. The neighbor kid looked at her and said "look, she's right there. She's not in the hospital. You're lying." He then proceeded to tell his mother and several other neighbors that I would not let my dd play with him because he is of a different race, and that I was lying about my dd being sick just so she didn't have to interact with him (of course he put it in much cruder terms). His mother instantly assumed her son was right and was so angry at us. She accused us of all kinds of things.
So I would encourage you to focus instead on helping your daughter (and yourself) learn how to deal with a world that may not understand illnesses and conditions that aren't physically apparent. Teach her to advocate for herself, in appropriate words. "I have a condition called Cystic Fibrosis that makes it hard for me to breathe" or something like that. Teach her that some people will disagree, or even worse, that they will accuse her of lying or making excuses, but tell her you support her and believe her and will always do so. And her doctors believe her. And all the other CF patients in the world believe her.
As for your in-laws, I would say something like "I'm sorry that you don't support your granddaughter in her fight against such a serious disease as Cystic Fibrosis, and that you don't understand what it's like to live with a chronic illness. Please read this [and give them a printout of the spoon theory]. We look forward to spending time with you when we are able to, but if you cannot understand and empathize with our physical limitations, we will be unable to join you for some family get-togethers. We hope you will become educated about Cystic Fibrosis [and give them a simple pamphlet or printed explanation] and defend your granddaughter in her fight to live a happy life." (I'm focusing on your daughter here, not to underestimate your own conditions, but often grandparents have softer hearts toward their grandchildren).
It is possible that they don't understand MS, Crohn's and Cystic Fibrosis, and that they are from an age where if it's not cancer or a war injury, or something in simple words like "a broken back", they just assume you have something that's like a cold or a bad pimple. Ask your doctor or search the web for how to explain your 3 conditions to a non-medical person. Make sure they comprehend how serious your health risks are, and how smoke or germs can make things worse.
I'm sorry you have to deal with this. My own brother has said I have "created" my dd's diagnoses in order to "get out of things" and "get attention". It is very hurtful. But we do all we can to verbalize to our dd that we believe her when she says she's in pain, and to re-iterate that her doctors are well-educated and that they support her and understand her illnesses. She is learning to trust them, and not the stupid neighbor kid.