Hence my distress, naturally. I realize the closeness, and it displeases me, but I don't know what else to think.
To me... I mean, it's like pedophilia. It feels like pedophilia. You knew this person as a small child, you knew them intimately, you don't get to have a romantic relationship with them. In the case of, say, long lost siblings? I've got no problem with that. If you never knew your sister, and she shows up when you're 18, and you like each other, I'm fine with that. But it strikes at the core of the moral fiber that you shouldn't be sleeping with a person who raised you, or was raised alongside you.