Responses

The older bro's parents are the father and his first wife (who already passed away) I believe. While the uke's parents are the 2nd wife (who is sick atm) and her lover. The uke and the sister are half siblings because they have the same mother. She is also half siblings with the older bro because they have the same father.
Spoilers....
Neither his “father” or “brother” are blood related to him. So it really is not incest. There is rape and blood though so drop it now if prefer not to read it.