Siblings hijacking inheritance - how common is this?

My wife’s mother and uncle got their inheritance hijacked by their sister. The sister lived near her father and as he lay on his deathbed she coaxed him into changing his will. All the while, my mother in law was taking care of their mother who had dementia (and continued to care for her for a couple years after their father’s death). The theiving sister got nearly a million $ in cash and assets and the other two siblings got about 15k each. It seems crazy to me, but both siblings still talk to their sister and visit on holidays as if nothing happened. I’m an only child, but sibling or not, if someone acts this way, they would be dead to me.

On my wife’s father’s side, there are 5 siblings and the only female sibling did her brothers even dirtier. She’s a total loser and her and her spouse and kids lived with her parents when her father died (not in a care-taker role, they were supported by her parents). Her mother had dementia as well, and this witch changed all the locks, hid the will (denies one exists) and took everything. The brothers can’t even visit their mother any more. She’s completely isolated her family and the sick mother from everyone and won’t answer the door if they come to the house.

I understand a dog-eat-dog mentality, but tradng your family for cash seems like a bridge too far. Is this kind of back-stabbing common among siblings when parents die?