H.M.
Listen carefully, because this is GOOD advice. :)
Whether or not to vaccinate is, and should continue to be, a personal decision that lies with parents. It's not your place to judge where she has obtained the info that has led her to this decision. If she's not asking for your help with this, don't offer it! Some moms get very defensive when they feel their parental rights are being intruded upon.
As far as being concerned with the health of your niece, don't be. There are tons of children out there who are not vaccinated and the chance that they will actually get one of the diseases we vaccinate against are actually lower than the chances that a child will be somehow hurt by a vaccine. I was not vaccinated as a child ad I am far healthier than my husband, who was, and most adults for that matter. There is currently little to no research to support vaccines that is not paid for by the manufacturers of the vaccine!!!
I'm not trying to start a vaccination argument, I'm just trying to ease your fears about your niece's safety.
Remember, when it comes down to it, we all need to be left to make our own parenting decisions. Hopefully they are well-researched. But even if we all do the same research, we will all come to our own decisions of how to apply it to our family. If you want to share information with your sis, and she is willing to listen, go for it! But if she's not, you're doing more harm than good, as she's most likely not going to change her mind.
Good luck!