No verse of Scripture identifies any of Joseph's biological children or of any cousins of Jesus.
2. Unless all of his siblings predeceased him (unlikely), a Jesus with younger brothers or sisters commits a sin from the cross in John 19:26-27, by relieving them of their obligation under Torah to support their mother
Only John was present - no other Apostle, or the brothers who "didn't believe in him".
3. Here are some opinions of sola Scriptura Protestants, who may have known their Bibles even better than Matt Slick (!!) on the subject:
Opinion is not necessarily fact.
I've said it once, I'll say it again: After all, what is Tradition but Speculation with a pointy hat?