What was your thank you note to the bride's parents about? Did you stay overnight at their house for the wedding?
In any case, your note back to the parents won't be another thank you. You will just be responding to the stepfather mentioning an upcoming visit. Even if you say something like "thank you for offering" somewhere in there, there won't be any reason for him to send you a thank you note back. So, you won't be continuing a spiral.
I'm not sure what you mean by it being bizarre that your son's teachers hugged you after giving them a thank you note. If it's that it made you uncomfortable, I wouldn't let that stop you from sending thank you notes to other people.
ETA: I saw your response that was made as I was posting. I'm not sure if it's common for people to send thank you notes to the bride's parents for hosting the wedding, so I think the stepfather may have felt it was proper to reply as if it were any other type of letter.