What bothers me about the ending of Oreimo is that if you look at the series as a whole, the last 3 ova episodes SHIT ALL OVER IT. If you stopped watching Oreimo at episode 13 then you did yourself the biggest favor. Where the anime ends is the perfect open-ended scenario where Kyousuke's future with his sister looks normal, and he has a chance at love with many different awesome girls. It keeps the progression of the anime's story intact, somewhat, and we can rest easy knowing that all the incest innuendos were just that; innuendos. But that is NOT the case with the 3 OVA episodes.
People are quick to forget how this anime started; a girl with a dark and weird secret tries to come out of her shell to make friends with other otakus, with the help of her brother who takes many socially awkward bullets to help her out. They make friends, accomplish things, and reach new levels of understanding that demonstrate how while otaku lifestyle can be weird and disturbing, the people themselves are good people. Kyousuke and Kirino made new friends, shared new experiences and gained romance. All these things were great, the show had many funny episodes and touching moments. Even the siscon element of things was more along the lines of over-protective brother than anything else.
But this ending comes along to shatter all of that, and not cause of the incest. Personally, if this was the obvious natural progression of the anime, NOT the light novel, then you can't blame the viewers for being shocked. Many people say how the anime's tone and direction ignored alot of the hints that Kyousuke loved his sister and the anime was more about making lighthearted jokes about incest; it was always kept vague but funny. If this was the obvious goal of the show from the beginning then I wouldn't have been bothered, but the fact that in episode 11 of the original season the title of the show was kinda fulfilled: Kirino said thank you at the party, a genuinely cute moment that showed that she was happy that he helped her make new friends and cope with her weird hobby. It was a moment that showed that they'd all developed friends and learned new things. So now, we come to the ending of the second season, where drama after drama finally results in 1. Kyousuke breaking the hearts of all the girls who were interested in him, 2. losing his longtime childhood friend probably forever, 3. creating a short time FAKE relationship with his sister, all to 4. NOT END UP WITH ANYBODY. I do understand that people say it's open ended and perhaps they're still dating in secret, perhaps.
But truly, you can't look at this series as a whole and be content with how this went down. All these people who wanted this and that match-up to happen seem to lack empathy, for the way Ruri screamed, for the way Ayase was devastated, for the way Manami was shocked. All for what? A fake relationship? People, they formed all these bonds, made all these friendships, to practically ruin EVERYBODY'S LIVES. See, I'm not angry at the characters, cause at least they admitted how fucked up they were and Kirino even lamented that her selfishness caused Kyousuke so much pain and it's affected his friendships. I respect and understand that people wanted Kirino to end up with Kyousuke, fine, but it didn't even go all the way, and their friendships are forever damaged by this event. To me, that's the really unfair thing, to watch their happy ship of friendship to go down in flames, not even cause of incest, but because of letting all the girls down for a temporary relationship.
Sorry for the rant, but I really wanted to provide some perspective for this. TL:DR- I'm not happy that all the friends they made and times they had were pretty much for nothing in the end cause of what they did.
Read more at
http://myanimelist.net/forum/?topicid=650423&show=120#oFSKmzGv4uL6dHeC.99 This is probably the most accurate description of all the flaws with the ending I've seen. It was absolutely disgusting and painful to watch.