Just having watched the last episode, here's my take:
No, she (Kirino) doesn't have sexual feelings for her brother (nor he for her). She just loves him as a brother, but won't ever admit it to anyone. Why all the hostility? Um, brothers and sisters are sometimes like that. In the end, she really DID want him to tell her to stay, but she couldn't just come out and say it. She didn't want to go either, being torn by the possibilities and leaving her family.
The jealousy? Ever read "Breakfast at Tiffany's"? (Don't have my copy at hand, so I'm paraphrasing here) "That kind of love can invite jealousy, too."
I must say, though.. I've never come across a more, um, constantly negative character since Tsukishima Aoba...
I suppose the second half of episode eleven makes a lot more sense once you've finished episode twelve.
Iskandar