There is also a wonderful site called
www.highconflictinstitute.com run by Bill Eddy, a former social worker now lawyer, who has written extensively on what he calls "high conflict people" (HCPs), especially when they tangle with the legal system.
I believe his institute is located in California, and you may find a lot of good material about how to proceed.
If your sister has strong narcissistic traits, it may help to hold out at the highest water mark so that when you do settle somewhere in the middle (the goal all along), she feels she has won. This may be a bitter pill to swallow, and it can also be the middle way that allows her to move on, feeling that she is not inferior, which can be the gas that fuels the battle.