Even if that's so, it doesn't mean that "brother" always means "cousin." (Jude calls James his brother. Does that mean they were just cousins too?) And it wouldn't make sense for it to mean "cousin" in the context of those verses either. For instance, why was Mary be hanging around with a bunch of her nieces and nephews in Matthew 12?
Also, the scoffers of Mark 6:3 were making a point that Jesus didn't have the authority to teach because He was just some carpenter's son like these others. Listing Christ's cousins would be irrelevant to that point, especially since Jesus' actual cousin, John the Baptist, also preached boldly and was the son of a priest. But John is not named as Christ's brother in Mark 6:3, or anywhere else in Scripture.