I would have been upset too - Like a pp said, cavitity germs are actually contagious
http://www.everydayhealth.com/dental-health/oral-conditions/0401/tooth-alert-are-cavities-contagious.aspx
Also, 60-90% of adults have the cold sore virus. Arguing that you've never had a cold sore, does not mean you don't carry the virus. It just means you don't react to it in that way. But the baby could react to it, and what a bummer to have cold sores at such a young age.
She didn't need to yell though. You were clearly just trying to be a good aunt. Let it go, and hopefully she will too.