I also have problems with this, as it makes the galaxy feel really small.
However, making people related to each other has a long history in epic story telling. The Bible has many examples.
Founding fathers: Abraham and Isaac? Father and son. Jacob? Head of a different tribe, but fuckit, he's part of the family too.
The twelve tribes of Israel? All brothers. Why not. It'll make politics easier if we're all related. But Zebulun means "dwelling" or "home" and isn't even a human name? Who cares. Let's just say the tribe once had a founder, and now Zebulun had 11 brothers.
Jesus and John the Baptist, both figureheads of similar but rival sects in Palestine? Let's say... cousins? Sure. Great. Makes mixing them into a single religion easier.