Dick and Donna view each other as brother and sister, and that's all they've ever been. (Dick mentions at her wedding that he loves her more than he probably would if they had ever been lovers and she agrees that she feels the same)
Marv Wolfman has mentioned that he wanted to explore a Dick/Donna relationship after Dick broke up with Kory and Donna broke up with Terry, but it never went anywhere as a notion and thank heavens for that (IMHO they are essentially too similar in personality, goal and aspirations for that to have been particularly interesting)