That would have made a good storyline for one of the sequels. Movie 8 would have been interesting if Spike had actually met his biological parents, and had to choose between going with them or staying in the valley. Alas, the writers weren't that bold, but perhaps they realized this would have worked and wrote and similar storyline for movie 10.
Do you know in one book for the original Spike met up with his parents at the valley? I don't see why this couldn't have been done in the movie, seeing as there were no plans for sequels, so it wouldn't have affected 8, and I like the gang better just as five friends, w/o any of them being siblings. Just my opinion.