You tell your parents to have more respect for the immense effort your sister and her husband are making to ensure their children receive safe and environmentally-friendly foods. You reiterate that difficulties conceiving will turn any parent into a very perfectionist/anxious sort of parent. That this aspect of your sister's personality was always there, and that's why she's a medical professional, for goodness' sake! You need to be perfectionist and slightly anxious in their line of work! My husband and I should know
You tell your sister that you admire all the work she does to make her children's lives as safe and sustainable as she can, but that you fear anxiety has clouded some of her decision-making, and that being self-aware about her or her husband's own anxiety might help with having a more balanced approach to parenting. Specifically, ask what her reasons are to disallow sleepovers. I'm sure she has her reasons. I was a very organic/safety-oriented parent of small children myself. Letting her verbalize all her arguments might lead her to realize, at some point, that she can relax a little bit.
Or this mediator/therapist role could backfire on you, and your entire family will turn you into the scapegoat
I've been in this therapist role myself, for both friends and family, and it's always worked out, but the people involved were willing to hear the other side of the argument.