Salam Ustadh, We want to visit a potential for my sister, as they talked already one time in person, my mother says she should not come with us when visiting the potential because this would decrease the “honor“ of my sister (the regularly talk on phone). Is this how it should be done in khitbah?

I did not understand the talking on the phone part. What is culturally accepted is different from one culture to another, based on traditions and so on. However, we should know Islamically that they sit together once or twice in order for him to see her and talk to her, and she as well sees him and talks to him. This must be in the presence of a mahram; they are not to be left alone or be able to speak alone on the phone or by any other means. As for who visits who, normally the man and his wife would visit your house, and they would sit with you and the sister, and so on. However, as I said, from one tradition to another, things might vary, and Allah knows best.