Well...they are step brothers....not related by blood so.... Zero
Some people will still think they're family (connected from paternal). And some will think they're not... usually the one with their maternal thingy culture. But after all, it's just some reading stuff. Nothing right and wrong in fiction...
Okay pause… I just read chapter 1 and my mouth fell open WTF U MEAN BROTHER!!! Wtf did I get myself into