It's nice, but lightweight and with a maddening plot hole. It also has underage fanservice.
Haruma is a university student who's always wanted a sister. When he was young, his mother had a miscarriage that went badly and possibly killed her. (It's unclear.) Haruma even asked Santa for a sister for Christmas. He's since forgotten about that... but Santa hasn't. Many years later, she shows up with a girl in a bag. (Yes, Santa's female. And a biker.) This present is that sister Haruma's always wanted. She's somewhere between 10 and 13 years old, perhaps? She's also a magical being, although that's easy to forget. She knows nothing and doesn't even have a name. Whenever introducing herself, she'll always call herself "my brother's little sister". (Haruma calls her Choco.)
The show's about Choco. She's irrepressibly nice, loves her brother and refuses to believe that anyone or anything she likes could do something bad (even dangerous carnivores). She has a notebook of information on how to behave, some of it age-inappropriate. (Choco's gullible and has no nudity taboo.) Everyone likes her.
The whole thing's pure fantasy, obviously, but still a bit too blatant in how it selectively ignores real life. What about school? Haruma never sends Choco to school and no one ever questions this, throughout all 24 episodes. Haruma himself goes to class. Choco's friends go to school. Choco, though, never does. She's like a small housewife. She'll mooch around the flat, go shopping and so on.
Perhaps sending her to school would have been impossible? Choco wouldn't have had a birth certificate, unless Santa had magically provided that too. Officially, she wouldn't exist. However you'd think that a school should have taken her anyway, while the show never suggests any other problems due to insufficient documentation (e.g. health insurance, dental care). In practice, the show doesn't want you to think about any of that... although it does acknowledge the strain on Haruma's finances. He's constantly taking part-time jobs and can't afford the luxuries that he'd have bought himself otherwise.
There isn't much of a plot. Choco and Haruma don't change much. Their relationship and circumstances stay roughly the same throughout the series. The running time's spent on:
1. Pleasant slice-of-life incidents. That's most of the show, really.
2. Developing the cast. We meet friends and flatmates.
3. Unrequited love. Almost every significant cast member is in love with someone who can't or won't respond. (Apparently the manga implies that Haruma eventually married the nice lady who's in love with him, though.) The main exception is Choco, whose love for Haruma is innocent, despite silliness like the naked apron. The show's relationships are a love chain, not a love triangle. Yurika will fall in love with Kakeru, who'll be in love with Choco. (They're all the same age, incidentally.) Makoto often shows lascivious interest in Chitose, who doesn't realise because Makoto's female and a hard-drinking party girl who's never serious about anything. (Makoto even proposes marriage to her in ep.13, albeit jokily.) Also, Chitose loves Haruma, who loves Ayano, who loves Kazuya, who loves her but is such a self-sacrificing idiot about it that his behaviour is a reason to dump him. (That was an annoying story arc.)
Only one of those relationships gets resolved. The others just... linger. It adds a melancholic undertone to the jolly light-heartedness.
For my money, the real main character is Yurika. She's a near-terminal tsundere, being obnoxious to all and sundry. Even in the last episode, after a ton of character development, she's still rude and sneering when invited to Choco's Christmas party. (Her growth comes in the fact that she's aware of it and, afterwards, feels guilty.) In her early episodes, she's often the horrified victim of accidental fanservice scenes. This is funny. Eventually, though, we realise how vulnerable she is underneath that thick layer of "dere-dere". She's actually sweet.
Chitose's lovable in a different way. She becomes the apartment manager, despite being a dozy goof with no self-confidence. She likes Haruma a lot almost immediately, but would probably grow old and die before ever managing to do anything about it. She overthinks things.
Then we have the fanservice, which is a reason to avoid the show. Most of it's from adults, admittedly. You'll see lots of harmless nipples. However the show's pandering to a very specific audience with the panty shots and more from Choco and Yurika.
I also got sick of the nekonyan dance. It's mildly cute in the closing credits, at first, but gets done to death when it infects the show itself. It's on TV and Choco's obsessed with it.
The last half-dozen episodes are a bit more worthwhile. Makoto's ep.19 conversation about her past was actually good. Ep.20 (the escaped panther) is Choco's funniest episode, until it turns dark. I also liked the gentle look at everyone in the last episode, including a little cameo for the dead.
It's a nice show. Everyone's kind and thoughtful, in their own ways. Sometimes it's funny. I liked Yukari and her maid with the eyes of a psycho killer. There's also the fact that Choco's secretly insecure about her status as Haruma's sister and will on two occasions seriously wonder if he's got bored and is about to discard her. However the show's also low-octane and a bit pottering, being more interested in character development than in plot. There's a lot to like here, but nothing that'll blow you out of your chair. On the downside, though, there's also lolicon-pandering and the fact that no one (including teachers!) ever asks why Choco isn't in school.
I watched it all. I'm moderately fond of the cast. It's not a keeper.