Yes, it was inappropriate and wrong. You are not blowing it out of proportion, you are a young lady now and he had no business touching you in any way. I hope your mother will believe you. If she does not, you are going to have to find a way to keep away from him, and also I am worried for the other kids. I am so sorry.
That is EXTREMELY inappropriate. Please, tell your mother! I have 2 kids, one is my step son & the other is my biological daughter. I would never do that to either of my children. And if anyone ever did that to one of my children, I can't even put into words what I would do. Please, please tell your mom!
Tell your mom he made you uncomfortable. See what she says.