He did an interview about a month ago explaining that he never actually met his biological father, interesting situation.
if thats true then this isnt much of a surprise tbh.
I mean I have no idea what the situation is but I would throw a guess out that his father possibly either had an affair or simply didn't care much for his son from a previous relationship and therefore made a new family later on.
I wouldnt think his sister and brother are full siblings but rather just just half brother / sister from his father's side. If seth has never met his real father then it wouldnt be too hard to believe that he never knew about other people his dad fathered. The guy is essentially a stranger to him.
If seth has had no relationship with his father then its perfectly understandable that he wouldnt of known about the brother and sister. It would be a different situation if they all share the same mother and father.
bit random for him to post this picture if this isnt the brother pictured......why even post it in that case?