First off, I am really sorry to hear your sister got kicked. I hope she isn't seriously hurt.
Secondly, it's very hard (I didn't say "impossible"!) to stop mares from kicking - it's a built-in defense in them.
Police horses are taught not to kick out because in a crowd that would spell disaster, but I think they do a lot of that training while mounted and they are extremely well trained to begin with. (Also, don't think they use much mares, but I could be wrong there!)
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Furthermore, your mare wasn't really kicking your sister - even if she got in the way - but was out to teach the other horse a lesson, and that is part of what horses do because they are horses.
Others here may know better, but I am hesitant to say that you can train your horse not to kick in a situation like that - but you can do your best to avoid those situations until she has accepted the other horse better, or been taught to refrain from kicking.
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Obviously, she has been bullied before she came to you, and now she is doing everything in her power to keep that from happening again.
Who gets to eat when, is a big hierarchy issue for horses - a clear way of showing who's the boss and who's not.
If she doesn't act up towards humans when being fed, you have a big advantage right there as her need to defend her food isn't directed at you.
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I don't have a good solution to the entire problem - and either way, I think it will take lots of time whatever you choose to do - but may I suggest a few things that might keep you safe in the meantime and minimize the problem?
From what I can tell from your story, Rin is very unhappy with the other horse being close to her food.
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Is there a way you can feed her on her own, or in a space that's removed from Ben? What's the smallest amount of space between them that she is willing to put up with?
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If no other space is possible, can you hang some sort of blanket or box curtain between them, so she can't see him? (Make sure it is well secured! Flapping or falling down can cause other panics!)
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Horses have a distinct pecking order. Make sure you always feed Rin first, so she doesn't have to be agitated over that as well.
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When you let them back out - and I would advice not doing this alone - can you take Ben first and lead him some way into the field so he isn't right by the gate? Or, if you feel Rin is easier to handle going out first; can you perhaps keep her away from the gate until Ben is out and the gate is closed? If you are two, the other person can take the first horse alongside the fence to a safe distance and hold it until the other horse is inside - then release both simultaneously, preferably facing the fence, so they don't just shoot away and knocks someone down.
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If you feed them outside, make sure you make at least three piles of hay, with ample distance between them, so your older guy always have one to eat from. I doubt she will try to defend more than two heaps.
Grain is harder to feed outside, unless you have two buckets and someone who helps to hold the horses until they are both finished.
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Sorry this was long and not a direct answer to your question, but I still hope it is of some help. Good luck, and, above all; stay safe!

Thank u I think that well help a lot I'm going to try that and see how it all goes. I know it going to be hard and take time and something I'm going to have to stay on top of.. thank u for your help. And my sister is fine... thanks for asking. ..