My DF has the vaccine every year (he has no spleen, so has virtually no immune system) and, like AnthonyBlanche, he suffers from side effects - the pain going up his arm and into his shoulder, and he gets bad head colds a few days after he's been jabbed. I presume this is because the vaccine is live... but I don't know. My DS gets bronchitis every year, and I have chronic and permanent sinusitis - our GP also nags us to have the vaccine, but so far... because of the way my DF has reacted to it, every year without fail, for almost a decade... it's not happening. And I've actually had flu... twice... and felt like I was dying both times (one lasted for about 24 hours, the second bout was with me for a week), and watched my DS endure Swine Flu - which he had Tamiflu for - and its aftermath. Until more research is done on the vaccine and how it affects the body, I'm not touching it with a bargepole - and I'm not anti-vaccines, either. Both of my children had all of their jabs, including the MMR, and I'm in full support of there being one to protect against chicken-pox (who in their right mind would wish to inflict a form of herpes on their child?!)
My DF is in his late 60s. He is an adult, and he can choose what chemicals to put in his body, if any, and when. His body, his choice.
I suggest you start to realise that your parents have the same rights as adults. Let them get on with it. Maybe they had their own doubts before your sister stuck her oar in. Perhaps they didn't. But they're not children, and they should be afforded some respect for knowing how to make decisions about their own bodies for, and by themselves! They're presumably of sound mind...? Let them get on with it!