I've read a lot of manga, but I think this is the only one I'd really consider awful.
First off, the plot is just a mess. It makes absolutely no sense most of the time, and is more like a bunch of random dramatic ideas strewn together with no consideration for keeping any sort of flow or pacing. I still don't understand the part where the female lead was going to be like his mother and teach him a lesson? That dissolved pretty fast though when other drama started, and it was just never mentioned again. I kept reading just to see if it got better, but every time one thing was about to wrap up, a new crazy thing got thrown into the mess.
Second, the characters are all annoying. I honestly have no idea how the female lead falls in love with the male lead. I don't think he's nice to her even once, and tries to literally rape her on multiple occasions (only succeeds in molesting her). The brother is annoying beyond all reason and his actions don't make sense to me, since they were siblings for the first 14 some years of his life. And none of the characters have any consistence at all in how they react to situations. Makes no sense.
And if you're reading for smut, it's not even great for that. Most of the smut consists of him trying to rape her, or at the very mildest more or less forcing himself on her once they're together.
I suppose the only redeeming fact is the art, and I did keep reading painfully to the end. So I'll give a 4/10, but unless you're really really bored, find something better!