How can Collin Creevey be Muggle-born wen he AND his brother went 2 Hogwarts?

Blood status is determined por parents and ancestry. There was a magical ancestor, (perhaps a squib), in the Creevey line, and the magical gene skipped a few generations, then came out in Colin and Dennis. They're muggle-borns because their parents didn't have magic powers, but no witch or wizard is ever really a muggle-born.

well maybe it depends on the family or it's just random. I don't think it was ever stated that for muggle borns only one of their children could have magical powers.