Your main question asked if you could sue anyone. I don't see any grounds for that. As for the other parent's claim, I cannot state whether this one incident is sufficient to constitute harassment, but I would consult with a local criminal defense attorney about that. This should be a lesson in being careful in what you say. Although the sister heard only parts of your conversation and took it out of context, one could construe your comment as a threat. And should an adult be saying anything like that about a just-turned 18 yr. old? I am suspecting that nothing will come of this, but consult with an attorney to be safe.
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.