Betty was never really considered to play his sister. It was just a cover story to hide the fact that herself and Bruce were having an affair.
As what was mentioned by our fellow member dragonb when he posted on Shaolin Chamber, GOD was not properly completed because Raymond Chow saw an opportunity to make a movie that served as a warning to anyone who dared to challenge the establishment like Lee did...
http://www.shaolinchamber36.com/kungfuf ... ent-200732
Given that the pagoda footage (including the bridge journey) would take up an hour of film, it's possible that the only thing that needed to be filmed were the non-action scenes which explained specifically what the treasure was, when this particular group of allies were put together, why Hai Tien didn't reach the final floor, where his family would end up after the mission was completed and how come his two remaining allies look at the Hapkido guardian as if they know him.
If you remember, Clouse was told to use as little footage of Lee as possible. Also, remember that Ng-See Yuen only made GOD II (a.k.a. Tower of Death) because he was told that Lee had shot more scenes than what was seen in the Clouse movie. As Linda Lee said in her first book about him: "As the movie now stands, only the fight sequences have been shot."