Well, it's a thing in real life you know ? There's girls, mostly young ones but not only, who really think that anal sex is not sex ; some of them even think that it don't goes against their purity vow. I even knew a woman who thought she wasn't cheating on her husband since she only do anal.
It goes for anal sex like it goes for everything, you find people all around the spectrum of possibilities. Look at Asian porn by example. It's not exceptional in it to see girls saying things like, "no, you don't deserve my pussy, you'll just have my ass". For a westerner it's surprising since, culturally speaking, for him anal is seen as reward sex for the guy and punishment sex for the girl (side note in reality it's neither).
To go back to your sibling thing, personally it don't disturb me. That an sister prefer to just give her ass feel legit and cover a lot of possibilities.
It can depict the fact that she feel guilty, or isn't this sure of her. Preserving her hymen is a way for her to lie to herself, she's still a virgin so she didn't had sex with her brother and nobody will ever know what happened. It can also depict her innocence, like I said a lot of young girls really think that it's not sex. It can also depict her perversity by adding a taboo on top of another one, or her submission (whatever by love or fear) to her brother ; she tried to resist, but comply in the end. It can even reverse the whole scene, and while the brother think she now own her, she'll trap him by addicting him to her tighter and most pleasurable hole.
So yes, for me it's 100% legit, but in the end it depend of the author and how he describe the whole scene.
