Imouto-ai is the strength behind many a great series, so it might bear more detailed consideration.
What is imouto-ai? Imouto-ai is when the protagonist has a loving relationship with a younger girl (or twin who's the exact same age) who (preferably) lives in the same house as him on a non-sexual original basis. This girl can be a real little sister, a step sister, a cousin, a couple of orphans who just happened to end up together, etc. In Seiken Tsukai World Break, the imouto only lived together with her older brother in their past lives together, and in this life are completely unrelated, but due to their having memories of their past life together it still counts. There are many paths to imouto-hood, but the requirements are always the same. The pair must be loving, and the pair must consider each other 'family' in some sense.
Sometimes imouto-ai is treated more like a 'father-daughter' pairing than a 'brother-sister' pairing, but the same dynamic basically holds either way, so long as they're only playing pretend at being father and daughter when in actuality their ages and power dynamics are much closer to one another than what an actual father-daughter pair would be.
Does imouto-ai necessarily have to be sexual? No! So long as the two get along well, the bubbly feelings of imouto-ai are definitely there. However, a tinge of sexual lust does tend to bring out the most fizzy feelings in the couple, so it's generally a good idea to include at least the hint of it. Just like yuri couples don't have to be sexual to be yuri, but a tinge of lust/possessiveness/jealousy/wishing to be together forever/physical intimacy does make yuri a lot more exciting and passionate than the bland 'friendship' alternative.
Imouto-ai is not found in The World God Only Knows. Elsie ends up only having admiring and friendly feelings for Keima, whereas Keima feels basically nothing at all for Elsie, and even spends a lot of his time abusing her in comic fashion throughout the show. Even though the series set up a perfect imouto-ai situation, and even hinted at it a bit in the first few episodes, ultimately it was just a red herring and the poppy, fizzy, bubbly feel of imouto-ai vanished into the starlit night.
Imouto-ai doesn't exist in Ranma 1/2, either, because Akane is Ranma's fiancee, set up from the beginning to be a girlfriend, not family. The relationship between the two is therefore subtly different from a relationship that is 'meant to be family' but somewhat 'slips' into something else.
Imouto-ai is not found in Ginga E Kickoff despite the main character having a little sister whom he gets along with swimmingly. This is because the two are rather distant from each other and barely interact on a daily basis. They may wish each other well, but they don't have any serious bond beyond that, at least that we can see in the show. There's no 'ai' in this relationship, no pop, no fizzle, just a normal family with a normal imouto in it.
The sheer number of imouto-ai shows that are in my rankings shows how powerful a tool this relationship is to build a good story around. Sometimes it's a sub-theme, and sometimes it's the whole theme like in Oreimo, but it's almost always a good theme to have around:
5. Sword Art Online
7. Code Geass
23. Da Capo
43. Angel Beats
45. Shingeki no Kyojin
68. Shukufuku no Campanella
76. Aishiteruze Baby
83. Usagi Drop
89. Akaneiro ni Somaru Saka
91. Scrapped Princess
94. Mononoke Hime
97. Amagami SS
104. To Love-ru
106. Grisaia no Kajitsu
119. Sora no Otoshimono
131. Seitokai no Ichizon
145. No Game No Life
28 great shows out of 170 have an imouto-ai dynamic going on. That's 16%! Why does imouto-ai appear so often, and why does it elevate a show's quality so much?
Imouto-ai, for one part, is a type of pure love that is stronger than any normal girlfriend-boyfriend pair. For one thing, they live together every day. For another, they have years of memories together. For a third, despite caring about each other so much, they have never had to resort to sex to prolong or sustain their feelings of closeness for each other. This means their relationship is stronger than anything the sexual bond can weave between a man and a woman. (This doesn't mean their relationship doesn't eventually become sexual. In Bastard, for instance, Arshes Nee and Dark Schneider do become lovers eventually, but he raised her for many years before that transition occurred, and it ultimately is not the core and fundamental foundation of their love for each other)
When a boy can treat a girl considerately and devotedly without receiving any sex in return, when he can restrain his lascivious impulses and just get along with his imouto as another human being, it becomes a very special, inspirational, empowering bond that everyone can admire and seek to emulate. This bond is all the more impressive if the boy and girl's feelings really are tinged with lust, but they manage to suppress those feelings and form a close platonic bond anyway. In a way, if we could just get away from the whole question of sex entirely and concentrate on solely loving bonds, the world would be a much more beautiful place to live in.
The world is extremely uninteresting when a guy does things for the sake of a girl in order to get into bed with her. Likewise, there is no duller topic than an ambitious woman trying to marry up into a family of doctors, vampires, princes, or whatever. Bizarrely enough, a lot of stories really are based around these most primitive of human impulses, which are not morally commendable or sentient actions in any way. You may as well write a story about bucks ramming each other for the sake of a doe. If, in contrast, you have a guy taking care of his little sister, with no ulterior motives whatsoever, but just out of the goodness of his heart, then you have something much more impressive to flaunt about humanity.
The greatest thing about imouto-ai is how it can slowly evolve into true love and become sexual/eternal. Da Capo is a perfect example of this, though it by no means happens every time. This is because the two started as friends, slowly got to know each other, developed their relationship carefully over many years, and only finally decided to live for each other as a final knot on their already long-lasting bond. There can be no stronger foundation for life-long happiness than staying with the same person you've been happy with already for much of your life. After living together for so long in a platonic fashion, they have proven even to the most sanctimonious saint how pure their love is, how strong it is, how deep it is, and how ready they are to take it to the next step. The fact that society condemns it and tries to prevent it only proves yet more how strong their love was, that they cared about being together even more than all the weight of society's condemnation put together.
Juuou Mujin no Fafnir also has a wonderful imouto-ai relationship going strong already. I suspect it will be joining my rankings soon enough, and 90% of the reason why is because of its great rendition of imouto-ai. It's hard to imagine a better relationship to write about, in any story, than the imouto-ai bond. You could say it's the easiest way to hit a home run without any quality writing skills at all. Just slap in an imouto into any show and have them bond a lot closer than usual and bam, you've got a masterpiece.
Shinmai Maou no Testament is also based around imouto-ai, but the overreliance on sexuality in the show somewhat dilutes this otherwise charming recipe. Still, it's stupendous that out of the 18 shows I'm watching this season, 3 of them would all be resting firmly on the foundation of imouto-ai. That's 17% of my viewing schedule. Tis the season to love imoutos, it seems. Well, in truth, every season tis the season to love imoutos~