I always understood the rule to be comma for identifying relative clauses and no comma for non-identifying clauses.

So, "my brother who deserves it" would imply I have more than one brother and this particular brother deserves it, while "my brother, who deserves it" would imply that I only have one brother who is therefore sufficiently identified by the expression "my brother" and "who deserves it" is simply extra information about my brother.

Since Father Steve presumably has only one father, the expression "my father" identifies who is meant and so the comma is usual.

Bingley