@Q W E E F A R O N I its bc he made his step sister ( i forgot her name) uncomfortable, so to things dont be awkward he left home (sorry for bad english)
Tbh, I don't think it's that absurd, at least in the culture where I live, it's even common for second or first cousins to date. I myself once had a crush on a cousin (chat, I was 8 years old, let's leave that aside). Of course, it's still uncomfortable, but it's something that really happens, and it's more common than yall think.