How sound is the Hadith: "Four men will be questioned about a woman's hijab: her father, her brother, her husband, and her son," and what should be said to relatives if its authenticity is confirmed and they reject the hijab?

There is no hadith (definition) stating that whoever does not cover his daughters or sister will enter Hellfire. However, the Sharia (definition) texts indicate that every guardian is responsible for his flock on the Day of Judgment. The father is responsible for his daughters and wife, the husband is responsible for his wife, and the brother is responsible for his sister if he has guardianship over her or can forbid her from evil. Enjoining good is done through advice and guidance, not through harshness.