A very noble wish and speaks of your character and integrity. It's sad how families can be broken due to greed. I know how that is because i've experienced it myself, seeing my family broken due to some illusion of treasure being left behind by the deceased.
They never wanted us to fight over these materialistic things, but fight we have. It's sad.
I hope you get your Mothers book and that your youngest sister gets her home.
Yes, it is sad how things like this can break a family, it is nothing new with this sister, she has always been a take, take, take type of person. If Mom would not have wanted the book back, I would let it go but I feel it is disrespectful to our Mother to let her keep it.
I am sorry that you have experienced this in your family, it is a terrible feeling that makes me have a sick in the pit of my stomach.
We are stronger for it.