How can I help my sister-in-law to understand important things?

I watch her 11 month old daughter during the week (my niece) along with my two children. Every time one of my kids or her daughter gets sick either with a contagious illness or not, she insists on bringing her daughter over. I have tried many times to stress how important it is to try to keep the children apart if at all possible when they are really sick. To no avail, she has a different belief system than I do, and was raised differently to say the least. My step son had pink eye last week and she brought her daughter over anyway, stating firmly that you can only contract pink eye from dogs and cats, not people, so that her daughter was at no risk. She was very irate about it. We have stickers in our backyard lawn (the kind that hurt your feet in the summer) and she insists that they are honeysuckles. She said that's what her mother told her when she was little and you can't tell her any different. (Honeysuckle grows on vines, not out of the lawn).

How can I help my sister-in-law to understand important things?
I would take her to your doctor and ask the health questions to him and let her hear the answers. Maybe when she knows the facts, she will realize that other things that she has been taught could be wrong also. Maybe you can direct her to the right book, or web page to educate herself about her unfortunate upbringing and lessons she carries with her today.