Even adopted siblings can get married to each other so I don't see how this has any problem. Asian countries even have a custom of adopting a boy to the household if they only have daughter. They will raise the boy as their child and as an adopted brother to the daughter but when they come of age, the boy would marry the daughter and inherit the family name. The way Western fans argue about this matter is rather weird. As long as they aren't related by blood, I see no problem. Like in Kagerou Project, 2 adopted siblings like each other and there are fans treated them like they are disgusting or it is real incest.