Plants that are siblings: How does this impact pollination?
In your case, your two bushes might well be half sib (the pollen might come from different donor) and one (or maybe even both) bush might result from selfing event. In any case, if the species is able to self, I don't see why full sib mating would cause any trouble.

Many species protect themselves from selfing. These plants are said to be self-incompatible. The evolution of the genetic mechanism underlying such self-incompatibility has been highly studied and is fascinating. The wikipedia article offers a good overview.