I am so sorry nlgh, I DO know how you feel. I have a half sister and a brother who did basically the same as your Nieces. I received almost nothing of my Mom and Dad's when my Mom passed away. I had finished the piecing of the quilt about a month before she passed and couldn't look at it for many years afterwards. I didn't bring it out til I met my new husband and moved away from everyone and everything I knew. It had to come out because I was moving. It reminded me how much I missed her.
You do know that there are people out there to talk to about possible parental abuse. It IS against the law in EVERY state. Your sis and her hubby have to be made aware that they do not have to endure that abuse and there are people out there that can help them.
Your nieces will someday get what is coming to them, as will my half sis and brother. It may not be today or tomorrow or even next week, but KARMA has a way of coming around just when they least expect it. It WILL happen.
Just remember your Grandmother and Mother in the best possible ways, heck someday maybe something they had will come to you when you least expect it. You will cherish it more than ever then.
Keep your chin up, and give us a big smile. Everything will work itself out in the long run.
:-D