Kim, Harry is Sophie and Kate's brother and he did not come back for Kate's party, he stayed wherever he is (USA or Sydney, I forget).
The person who returned with Sophie was Zeke. His dad Alex was married to Susan for a while when she and Karl were separated, but Alex got sick and died not long after they got married, and Susan continued to look after Zeke and his sister Rachel for years, even once she and Karl got back together. Zeke was on the show from 2005 till 2011, and when he left, the storyline was something about him going on exchange for uni in London for six months, but obviously it's been much longer than that.
Brad's dad's name is Doug.