How to make my family understand that our neighbour is touching me inappropriately?

My family has been living at this rural town since centuries and we know almost everyone around us. So this neighbour of ours, who lives 5-6 houses from our house is now in his late 60s/ early 70s and has very well witnessed my father being born, my parents' wedding and my birth. Now, there's a weird habit among people in this area, old people or grandparents often jokingly tell kids that they would marry those kids in future. This joke stems from the concept that once you're old, you return to your childhood and the children around you become your friends. As this man is my grandfather by village relations, he had the habit of joking about me marrying him when I was a kid. Now, such jokes are supposed to stop once the child is 6-7 years old. However, he didn't stop, not in my teen years, not even when I became an adult.

After completing school as I lived at different places mostly due to my studies, I didn't have to meet him often. However, recently that pattern has become more disturbing. Whenever he met me now, he caught me off-guard, coming stealthily from the back when I was paying no attention to my surroundings and then he'd place his hand on my shoulder, back or waist and whisper to my ears: "so when are we getting married? Why are you running away from me?" This happened yesterday as well, when I went out with my mother for shopping and as she picked things, I was bored and played games sitting at one place. He came stealthily, I was shocked to find a hand on my back and as I turned to my side his face was extremely close to mine. He said: "get yourself a wedding dress then". It was horrible, the sales-guy in the shop even noticed it from a distance and came to see if anything was wrong or whether I was in danger.

I have told him many times that I don't like it. But every time I'm rude to him my parents would get mad and say that these are casual jokes made by someone who is my distant grandfather and that I'm too sensitive. How do I explain myself?

How to make my family understand that our neighbour is touching me inappropriately?
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