Sorry about what happened to you. And so sorry for you loss.
The important thing is that your mother would be pleased that you were there. It was between you and her. No one else matters.
And another thing, even in the JW community your brother and sister in-law's actions are not what the society would condone.
They were off and marching to a different drummer.
If it were reported, they would be counseled for their actions.
I think you're correct in writing them off. What they did was unloving, vicious, cruel, unforgiveable and coming at perhaps the worst moment of your life.