You can't force your sister or her husband ..to accept what happened was wrong and sadly if they don't want you to have any part of their life or the baby's life...there is nothing you can do. You made a peace offering which wasn't accepted in a very nice manner. If your sister is not willing to discuss this on going problem with you...then you have to let go and get on with your own life.
I must say ...as a mother and grandmother...if any bf/husband spoke about any of my daughters the way he talk about you ...as your mother , I would have kicked him up the bum , and out the door...he would never be allowed back into my home until he apologised. I will never allow any sort of nasty talk comment about any of my family ...so my son in-law and daughter in-law would never dream of behaving in such a nasty manner. We may disagree about things..but we are not disagreeable.
Maybe it's time your mother stepped in and arranged to have an end put to all this nonsense from your sister and her husband. It just might help.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.