Both boys got their moms' look so we have no idea if they really have the same father or not
I highly doubt that they share the same mother it be weird for them not be living together if that was the case , I believe something happened to tall dudes mother then he remarried to blonde snob mother, this is my assumption therefore having no relation other than being step-brothers which is not incestuous.
Also resulting in some sort of grudge of being part even related to the dude who is unfaithful to his mother
I don't think that his biological dad, however I could be wrong.