I would say that something else may be at play here. There are no canon-cases of two muggle-born FULLY biological siblings. There are a few ways that situation like that of the
Creevey Brothers can present itself, including but not limited to the following.
It is possible that they may be step or half brothers, with one being a muggle-born and the other being a half-blood, through a different parent and it is not explicitly mentioned.
Like the McGonagall family, it is possible that one of the Creevey brother's parents is a witch/wizard that did effectively hide their power.
Many times in the wizarding world, Squibs were sent to live in the muggle world. However, they do still carry the magical gene and therefore can have magical children. It could be possible that one of their parents (or both!) was a Squib that still had magical children
Sometimes, if a witch/wizard is expelled, they lose their wand, which is snapped, and/or CHOOSES to have their memory wiped (prior to the First Wizarding War, I found this in some JKR interview) It may be that one of their parents faced expulsion and chose to wipe their memory rather than face embarrassment and had magical children, though this seems the most unlikely of all of my thoughts.
Of course, this also could be a plot hole on JKR's part, or they may be that rare case of two muggle-born siblings.