This story needs a yuri tag, or at the very least a shoujo ai tag. Literally the entire focus of the manga is the romantic relationship between these two sisters -- in fact they're pretty much the only characters you ever even see. I cannot for the life of me imagine why this would be left out.
Anyway, this is a fun, lighthearted story. The title pretty much says it all. There's not a lot of depth or contemplative moments, but it does actually get into backstory a bit, which makes their whole relationship feel a lot more believable (still an absurd premise but hey
The art's not great. Nothing to write home about. The characters aren't super fleshed-out, but they're not entirely one-dimensional either. The story is simple, but it's not trying to be complex, and it does what it's trying to do quite well.
Definitely worth your time if you're looking for a yuri story that imparts warm, fuzzy feelings. If you're not into yuri, you're going to give this story a 1, because there's virtually nothing else there.
P.S. Despite MAL saying there are only 5 chapters, there are actually 21 currently, and it's still ongoing.