I will try and keep it short.
FIL had an affair when DH was 18. He left the family home, breaking DH's Mum's heart. DH looked after her and his younger sister. A year later his Dad returned. My DH went travelling for a year to get away from it all, he found it difficult to welcome his Dad back with open arms. 4 years later his Mum died of cancer. My DH and his sister loose contact with his Dad. My DH just doesn't want to know about his Dad, has no respect for him, and will never forgive him.
13 years later, I bump into FIL, he has since remarried. I allow him to have access to his grandchildren, and become friends with his new wife.
My husband keeps his distance, but approves of the kids getting to know them.
Now, his new wife knows nothing of her husband's affair whilst he was married to DH's Mum. She can't understand my DH and his sister's behaviour toward their Dad and becomes understandably upset. She keeps trying to sort of force the issue and gets frustrated when my DH won't be all friendly with his Dad. She must just see my DH and SIL as being nasty to their Dad for no reason. He has not told her why they behave like it.
I can't really explain to her what happened before she was married. I think the Dad is making a fool of her because she says things like "He is as good as Gold, he would NEVER have an affair" Its hard not to do a 
I feel like my DH is being made to look like a shit, his step Mum is pretty disgusted by him, she thinks he is rude. Her husband will never put her straight, I can't. It is really hard to stand by and watch it all. It seems to me like his Dad is happy allow my DH to be misunderstood. I think its unfair on both DH and his stepmum.
It wasn't short was it
sorry.