I don't agree with the other comments that seem to think this is somehow too real or anything like that, it just isn't. The art is fine and it's established all the time that they aren't siblings. They didn't act any more like siblings than how people in shoujo manga tend to act towards friends. There's about one joke about porn in the entire work, and the dude acts nothing like a perv(and the manga doesn't go for gags often either).
This manga is basically is more about an age gap that creates a big bro type of situation except in this case, He is legally her bro because their parents had recently remarried before dying by plot. But he isn't her guardian, her grandmother is, and her grandmother let him stay.
It is a pretty light manga, it goes through all the plots a longer series might in a shorter amount of time but never feels like the story is rushing. There's even time for a side couple of sorts, the girl in that couple actually ends up serving a story role as someone who's a bit more impulsive and aggressive than the Misao, and they become friends and she has someone to talk with.None of the characters annoying or unlikable.
The ending is a bit forced in the sense that she suddenly decides she needs to prove(or the mangaka does) something.
It's like in Toradora, she decides to go far away to school. Fact is she doesn't have a career plan and I doubt that there aren't colleges where she lives.
Basically if you like shoujo manga series where someone overlooks the heroine because they've known her too long or think of her like a kid, as a sister ect, this is like one but in about 1/3 the time.