I'm more incline on assuming they're brothers by metaphorical sense. They had a glaring differences such as eyes, physical appearances etc.
Haven't heard about them coming from the same mother either. If some people are taking the word origin as being implication of them being tied by blood, I'm pretty sure they're wrong.
Origin does not imply coming from same mother. Origin means they both share the same point of origin. The origin being demon, they're both demonic being and as such share the same point of origin.
You and I also share the same origin, being human or Homo Sapien. We both shares the same point of origin, that does not in any way means we're immediate family, no, nein.