Offensive how?
'I am offended at how stupid this show is' or 'I am offended at this perverse incestuous storyline'
The former goes for pretty much the entire show post-S4, but in this case, it's the latter.
Not so much because it was presented as incestuous, but rather because it was presented -even in the show itself- as a "well, he's technically not really your brother" kind of way.
Which, honestly, isn't how it works at all. There's no distinction between adoptive and biological children in these families. There's no moment where you look at your sibling and go "oh hang, but you're not my REAL brother."
I can imagine such things happening in families where two divorced people with children from previous marriages get together and form a new family, sure. But families where your parents adopt a baby? Nope, that's as much a "real" brother or sister as any other.
The Dexter writers are just the worst writers. Absolutely terrible.
