It's extremely rare but it happens.
This National Geography documentary talks about it
9:45 but you might want to start it a little earlier (or watch from the start)
"Identical twins are almost always the same sex but surprisingly there are a small number of boy girl monozygotic twins."...
Akiko from OniAi. It would be an understatement to say she is a Brocon. She probably said the quote on that image.
About said image, does Kobato from Haganai on the top left really have feelings for her brother? I heard that before but there didn't seem to be anything like that in that anime...
Two Anthology chapters did the concept. One was Mihari and Her Partner like you said (or chapter 12 on here). Another one was Mihari (male symbol) which is chapter 28 here.
I don't see how wanting to "rub one out" every so often goes against the character growth. Mahiro's flaw was being a shut in with no friends not that the character had any desires at all.