I am now a salesman in building materials, working with a relative of my family. I have been working for half a year, but I feel that I am not suitable for this job.

that depends on whether you think there is room for development in this job and whether you are happy or not. If there is no room for development and you are not happy, then give up. As for the leader, he is your relative. I hope you will stick to it. Don't promise to come down just because you are embarrassed. You can find a time to have a good talk with him and talk about your thoughts. I believe he will understand and support you. I hope you can find a job you like.