I hold onto grudges when I don't have a sense of closure. If someone hurts me and I didn't get to have a chance to stand up for myself or have my chance to serve the hurt ball back then I will carry it forever, the rreal sucky part is it is me that holds myself back from closure and hitting the ball back more then anything. I am very nonconfrontational as well- so it isn't like I am going to search for it or make the closure happen either. Only with people I truely trust will I confront, that is new for meand have just started doing that. If I don't feel like the person will listen to me or acknowledge my feelings then I won't bother, I have to truely feel cared about and feel like it is worth the effort and risk. In a sense I have to feel it is worth risking the relationship ending if it will on the flip side make it into something better- I have to believe in the something better being achievable- If I believe that it will be better and yield more closeness rewards then it is worth the risk of losing in the worst case scenario- being completely honest with hurts in confrontation very very hard-even to the point of letting relationships die off from starvation of communication is easier if there is no hope of reward and better results. Getting the hope is key. Perhaps if she can be convinced of hope.
I feel so bad for your sister- because she has been denied a super sense of closure. Her father is dead- so she can't have the closure she wants with him and is probably tortured by it. I agree with others that now the anger has been displaced or double placed on your mother.
I think INFJ's need to know that they have a right to their feelings and need a kind of celebration of them or sharing of them. I wonder if you were to share in criticising your mother with her- or if you got her with someone who was criticsing their mother- mother bashing party of somesort- if she wouldn't feel better and actually even turn around and stand up for your mother.
It might sound weird, but sometimes I feel so much better and actually more objective afterwards when a friend will join me in a hate party as it were about someone or a similar someone- then it is like I see that I am not the only one, I feel understood, and I get a chance to see what my feelings and words look like on someone else's mouth and heart and then I feel compassion for the person being hated on and then turn around and realize they are not so bad and do not deserve this and I will become more understanding towards that person and will make sure the other person who joined in is on the same board.
I have to have a chance to air out all the hate and hurt I feel and just have the freedom and support to do so and after all the junk is out I can rebuild. Perhaps a hate party helps in that in that situation you can be completely honest and free to vent out everything. Fe is so strong that it will keep me from being as honest as I need to be in order to move on- I have to be in a situation where I am forced out of that and have freedom from those Fe sensibilities- to force out the darkest depths of feelings I may have when hurt. I can recover so quickly and so sun shiny and be the rainbow afterwards after I have been able to be the storm but my winds must be given a voice.
Yeah- it is like I need to completely demolish on the inside before I can rebuild anew. The thing that is dying needs to die and then I can plant and nurture the new creation. Her relationship with your mother might be dying and might need to completely die and something totally different might need to be replanted and a new relationship might have to be nurtured. They might have to start all over in their relationship and be something new.
It may take time. I think being around other people with the same type of feelings and hurts might be good for her. I wonder if group therapy is good for INFJ's in particular because of the need to be understood and related.
I don't know about your sister.
These are just my 2 cents and my experience.