@crazered I do not understand why you are upset. It may or may not be true about his teeth, but that remark should not cause any concern to you. Hopefully, your grandson has been to a dentist by this time and is taken on a regular basis. If not, you might want to mention to the parents that he should start going. A dentist would know if there is an issue.
I do not want to be unkind, but if you feel that you are being treated as a child, maybe it is because of how you are reacting to a remark made by your sister. Unless words/thoughts are untrue and/or mean-spirited, my family and friends can say anything to me. I consider that normal conversation.
The next time that I hear salt and ice together, it better be in a margarita!