Yes, I believe it's very likely authentic. Although Jacob and Joseph
were very common names during the 1st century, the referal to "the brother of.." is seemingly very uncommon.
Such osuaries were very common in the 1st century, hundreds have been found. Incriptions were common, among others several references to biblical persons, Jesus and signs of the cross (this dismisses the Watchtower viewpoint).
Take a look at several "back issues" at;
Although these findings do not, "prove the divinity" of Jesus,
they do say something about what position Jesus had on his followers unto their death.
Finn