So, your contention is that it would be totally impossible that the Holy Family might have taken in relatives' children who were orphaned? And I am not saying that this is the case, it is more likely that they were cousins who grew up with Jesus. The point is the notion that the Virgin Mary had other children was NEVER mentioned prior to a few hundred years ago.
As an aside, one problem that will always exist with the Gospels is the fact that although they were written in Greek, the words our Lord spoke were in Aramaic. And Aramaic DOESN'T HAVE A WORD FOR COUSIN.
The notion that she was sinless didn't come about until the 300s. But hey, whatever you want to think. Ignore the meaning in the Greek. Ignore common sense. March on your way little sheep. Just be aware, the wolf has a mean bite.
Oh, and before you say it, Irenaeus didn't say she was sinless. He said she was a virgin - which she was. Just not perpetually so.