love2Bworldly, I am so sorry for your loss. I know it's of little comfort, but you're not the first to see first-hand the way JWs treat family in times of crisis. And sadly, you won't be the last.
I think gma-tired2 hit the nail on the head. It's not so much that being JWs causes people to be selfish and greedy when someone dies and there is an "inheritance" to be had, it's that being JWs is no protection from being so. They can be just as greedy and crass as anyone else. But that does make such JWs hypocrites because they claim to be "True Christians" with standards far above those of mere worldlings.
Another common thing among JWs is to not inform non-JW relatives when a JW family member is seriously ill or has died. It seems they don't want the non- or ex- JW family to visit the sick person or even come to the visitation and funeral if there has been a death in the family. You know, they can't risk getting "aposta-cooties."
I am neither officially DA nor DF, but my own sister told me in no uncertain terms, "I am no longer a conduit of family information for you." There have been children born, infant deaths, divorces, and disfellowshippings in my family that I only found out about through "back channels."
Please accept my condolences. If I may offer a bit of advice... protect your own interests and those of your other family by holding your sister to account for the things she took improperly. Make sure your father's instructions are carried out to the letter. That's one way you can honor the memories of both him and your step-mom.
Take care.