Do you mean Oryx's brother or Crota's brother? Either way, Crota is spoken of as an only son among many sisters, and Oryx speaks of his other siblings frequently. The reason I think Nokris is Crota's mother is because: A: Family tree only lacks the mother figure to Crota. Oryx is incapable of producing offspring without a Wizard mate. B: If Nokris died Oryx might be somewhat sad about it(or perhaps disgusted by her weakness) in a Hive way, and be unwilling to speak of her but still keep her memory. I just find a (hidden)missing sibling to be more unlikely than the already missing mother. Still, there is no proof of it one way or another, so Nokris could be Crota's pet dog-thing for all we know.

HA! Hive lack obvious external genitalia, there's no way you are truly Hive!