What should someone do who undressed in front of his sister when he was an eight-year-old child, and feels distressed and saddened when remembering that?

A child is not held accountable for his actions, due to the Hadith: "The pen has been lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane until he regains his sanity." There is no blame upon your sister for looking at you at the age of eight, for a boy eight years old or younger is permissible for a woman to wash and look at his private parts.