[Cicero]

There are two issues. As you say, the word translated as "brethren" or "brothers" can mean simply cousins or blood relatives.

The other issue is that although part of the inscription seems genuine, there are problems with the "brother of Jesus" part. It seems to have been added at a different date by another hand. And another expert has said that there are inconsistencies in the spelling.

[infowarrior]

And another expert has said there are inconsistencies in the spelling...

Ever taken a look at the spelling here? That comment means very little...

the infowarrior

[EternalVigilance]

It seems to have been added at a different date by another hand.

The article that heads up this thread says the exact opposite:

But Israel's Geological Survey found that the patina on the stone indicates the letters were engraved in ancient times and were not tampered with.