Wow, please don't think this is normal. No matter how widespread, and I don't actually think it is, no one should have to put up with this and consider it normal. Sadly, when they're your family, or even if you live with people like this in your community, it's hard to just blank it out and avoid it.
I agree with Meta that this shouldn't be your responsibility. You're a sister, not a parent, although I commend you for being so strong and knowing that this situation isn't right while caring for the more vulnerable people in your family like your younger sister.
I think in a year, when your sister is 16 and legally able to move out and all the rest, it would be great if she could live with you. That way you could remove those still unpermeated by the violence of it all to start a new life. You won't be able to influence the others, no matter how nice that would be.
I honestly don't know what I'd do. Frankly, I'd likely move out and away, disowning them, if I had the means. Even if not I'd run away to a charity; there are many who deal with families coping with violence. I couldn't comprehend why anyone would think that violence was right in any form, and especially not in the current climate where if someone supposedly 'disrespects' someone by looking at them in the wrong way they could end up stabbed to death. In fact, if I ever heard anyone threaten to stab or harm someone I would avoid them forever, and possibly alert the police to the threat too. Selfish maybe, but it's self-preservation, and sometimes there is just nothing you can do to make things better.
As for your parents, I think they've given up hope and feel powerless. In fact, they're taking out their frustration on you because they know out of all your siblings you're the one that respects them the most, as parents if not as people, I don't know what your feelings about them are. But that's classic, to be angry over someone who has too much control over you (i.e. by threat of violence) and to take this anger out on someone more benign.