[IYAS9YAS]

I knew a girl in high school who had a "relationship" with an older stepbrother.

I know of a couple like that. The daughter of the wife married the son of the new husband.

IIRC, though, the son didn't live in the same house with them (lived with his mom).

As far as I know, they've been married more than 25 years by now.

[who_would_fardels_bear]

My understanding is that step-sibling relationships often involve the older sibling leaning on the younger sibling to the point of it being rape.

This was the case with the girl I knew.

Encouraging incestuous relationships is going to have the same effect as encouraging polygamous relationships: providing cover for exploitation.

Every once in a while an incestuous or polygamous relationship might turn out OK, but it is prudent for those in such relationships to keep their mouths shut knowing they are the rare exception.

[g.orwell]

I know of a couple like that. The daughter of the wife married the son of the new husband.

IIRC, though, the son didn’t live in the same house with them (lived with his mom).
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I see nothing wrong with that.