Slightly unexpected honestly. With the isekai's i've read, none of them have touched on "inter-family marriage" which used to be a thing in some cultures for the sake of keeping "pure-blood" or so i assume. Considering the fact that this seems to be common practice and there seems to be no genetic problems with the father, sister, brother, etc. Perhaps this world kinda just ignores the underlying problem of such marriages.
MC, being raised from the modern era, is definitely not gonna be ok with this.