This story gives a rich, dynamic, and ultimately realistic story (hence it is dynamic) to the struggles and sacrifices a teenager must face as a teenager but also as a guardian, de-facto or not, to a preschooler. The other difference to Aishiteruze Baby is that this is a female girl, Hinako, dealing with her three year old sister, Minori. In AB it was a male with a female cousin. In any case Oneichan to Isso is ultimately one of the best if not the best in realistically showing a teenager dealing with the responsibilities of an adult.
Although I found myself attached to Minori, mainly because I thought she was an utterly cute 3 year old and found her actions (if not her speech) was very typical of her age. I did not find the same feeling towards Hinako. However, Hinako is in character. She's a teenager who looses her mum at 13 to do complications in childbirth---so the mum died giving birth to Minori. The dad is there and very supportive but Hinako bears the brunt of the responsibility. Funny enough, both her a her boyfriend take on that role. The boyfriend is similar to the lead character in AB. He was very realiable and never had a cross thought about Minori, sure he wanted alone time with his girlfriend but he never miss treated Minori and was extremely kind to her. He's a great surrogate father figure.
We see Hinako though struggling with the role of being a surrogate mother. And this is natural. Both of them share a mother and unfortunately during the most crucial years of her life (teen years) she was robbed of the most important person who gives guidance. And at the same time, forced to play that role for her baby sister. It's hard and at times I felt she was too hard on Minori, but then thinking on it her reactions were natural and you can't feel any sort of love-hate with the characters. Just ultimately pity and respect that they're doing pretty well even with ups and downs.
This story has little romance as you are indoctrinated into the story with Hinako in a relationship. So the focus is entirely on her relationship with Minori. However, as a bonus the mangaka does an extra chapter to show a pre-school first love which was utterly adorable.
I'd say storyline is very good. The art was mediocre but Minori is adorable none-the-less. The characters are exceptionally well done. And I enjoyed it and think many others will also. So this is not a shoujo just a good story of family love, support, and triumph through tragedy.
