Zenitsu is very informal, excluding Inosuke, compared to most people. He refers to others by either nicknames or their first names. Plus, demons are considered a tale to keep children in at night back then. Now, demons are known as a myth that never existed in the first place. It's highly unlikely Kanata and Touko know that their distanced cousins. Tanjiro was never a prideful person and neither is Kanao, so the two probably didn't discuss their former jobs when they got married and had kids. Plus, they might as well as have enough money to support their family easily. Zenitsu and Nezuko are a different story. Zenitsu may boast while Nezuko just humbles herself. His children probably saw him as some hero while the grandchildren most likely didn't believe him. Inosuke and Aoi? I don't know man. Inosuke probably mispronounced their names. That is why Inosuke's great-grandson is alone.
Oh and Sumihiko can definitely see ghosts and hear them.