Hey
As a former grudge-bearing person (I prefer the word vengeful), I hope I can help you understand and help your sister. Nonetheless you know your sister and I definitely do not.
First off, you can't force anyone to learn. You can advise her; if it doesn't work out she has to learn for herself what is helpful.
The first lesson to be learned is holding grudges generally hurts oneself and does not hurt the one you hold a grudge against. My former roommate A and I would have an argument (often over something trivial) and then have to attend a social gathering afterward. He could immediately let go his feelings of anger and forget while I stewed. He would be able to have fun, while I suffered. Once we were private again, I would blow up at him and he would say "What, that's your problem for not having fun." and I would respond "I can't have fun when I am angry, and you know that and therefore it's your fault." This type of argument is petty and useless. It hurt me because I didn't experience life as much as I should have and exasperated the problem.
The second lesson is that the people you hold a grudge against will eventually for one reason or another disappear from your life. Your sister needs to realize that while mourning about her father, she does have her mother, who will eventually die. Sorry for just saying that.... My personal example is that my roommate did something moronic (I don't remember what), but it didn't matter to me, because he was going to be leaving in a few days and I would never again have his friendship as much as I did now.
Another thing I would like to say is that her sudden explosions are probably based off something that seemed trivial to you, but are important to her. Sorry, I do examples; I have yelled at roommate B after having to refill the soap dispenser. I always had to refill the soap dispenser, after asking him several times to fill it when it was empty, and I always had to put new toilet paper on the roller, and the garbage, and he inconsistently cleaned his dishes....this expanded more into everything he had ever done to piss me off...which was alot as he did some things knowingly to annoy me. Next time he said a joke to me trying to be witty and clever, I would yell at him "Well, you are lazy and intolerant." and then go to my room and close the door. If he asked me what was wrong, I would respond back with "You should know what's wrong." For me logical, for him the soap was trivial enough if he didn't even bother to refill it.
I'm sorry if this was long-winded and if I offended you. I do hope you have some success, and have enough patience and love to last through it.