It is perhaps ironic that there is a well-worn conservative Christian phrase, of Biblical derivation, which illustrates wonderfully a point that Earl Doherty and Neil Godfrey either are missing themselves, or are fully aware of but hope that their readers will miss, namely that in and of are not universally interchangeable or synonymous.
There are indeed some instances in which one could use either. There are many more in which one cannot.
I would request that Doherty and Godfrey either offer an instance of “sibling in” being equivalent to “sibling of” or otherwise concede that there is no linguistic evidence for the equivalence that has been central to their recent comments and posts, namely that “brother(s) of the Lord” in simply a way of saying “brother(s) in the Lord.”
If words and grammar are infinitely flexible, then there is no point in debating further, since there is no basis for drawing one conclusion over another. If they are not, then clearly this terminology represents a weak link in the mythicist argument.