I know, I know, it's got the word "Sister" in the title. I really should have known better than to even watch it. But someone has to make these sacrifices.
Yuu is a teenage boy who wants to write light novels. He drafts new books constantly, and is rejected equally constantly, lacking the ability to come up with truly great works. (And possibly even middling works.) If only there was some sort of inspiration he could rely on to make his writing better.
And yep, his sister is the solution, though not quite in the way you're thinking. (Please stop thinking like that.)
Suzuka, the sister of the title, has written a book of her own, having flicked through one of his efforts one day and gotten inspired. She's his utterly perfect younger sister, and that means she's a very good author. So good in fact, that she wins the first competition she enters, with the first manuscript she's ever created, which is immediately published to critical and public acclaim (and massive sales).
Unfortunately, since she's still at school and not allowed a job, she wants her brother to pretend to be her, and let her keep being a ghost writer. This has the other advantage of not having people judge her for having written a book about a little sister who is massively in love with her brother. Which she definitely isn't. Oh no.
So that's the central sibling obscenity set up. But we can't just stop at one creepy relationship, can we? This is one of those shows that has decided that horrible perviness is actually endearing, so let's have a few more women who are inexplicably obsessed with our protagonist. Firstly, he'll obviously need an illustrator. Perhaps with a silly name. Something like "Ahegao". That's suitably on the nose.
And after that, he'll want an editor, who can hunt him down in a funny manner when a deadline is coming up, possibly with ropes and chains. She might even be nice enough to help him with his writing by giving him experiences that he can then put in his books. That seems sensible.
Finally, he should probably have a rival. A fellow author who he can both admire and be admired by, who will spur him on to greater efforts while maintaining a professional distance.
And that's only about half of the screenshots I took in the space of two episodes. Honestly, there's worse. And even worse than all that, the animation took a complete nosedive between the first and second episodes. Whether it's due to budget reasons or what, it suddenly got awful. So I can't even recommend it based on visual quality.
Just don't watch it. Please. Save yourself.