D.B.
Not all siblings are best friends. You can't force that. I think your best bet is to insist on a level of respect and consideration. It seems like you feel the older one needs to do all the changing - not hurt the younger one, be more mature, keep cool, stop being selfish and oblivious, share secrets, etc. But what do you expect of the younger one? All you've said about her is that she lashes out when she's hurt - but that really says that the older one starts the problem.
Just because the younger one and you want for them to be close doesn't mean that the older one should have to change her personality and her style.
It's really up to you to not wish for a relationship that isn't there or force a closeness between 2 kids who are so very different, but rather to teach each of them to make equal effort to understand the other. The older one is an adolescent, and you'd have issues with her even if there weren't a younger child! The younger one will give you a run for your money soon enough. It's time to teach them some diplomacy and some ability to not incite or tick off the other one, and to work out their differences with respect. Neither one of them needs to go to every fight she's invited to, you know?