You confuse bad people when you give them gifts. It’s like giving a treat to a puppy after it soils your carpet. Giving gifts to your mother reinforced her denials, and allowed her to tell her sisters, “See! I was right! Nothing happened!” When your mother’s husband died, there was no opening to repair your relationship. He didn’t cause her behaviour. She did. You can’t give forgiveness to a person who isn’t asking for it.
Your mother has been telling the same lie for decades. An event ruptured your relationship. You are still caught there, and she sees no advantage to herself in admitting the truth. She chose a man over you. Some women do that. A man, any man, is more important than their own child.
Your beliefs tell you what to do when you abuse another. What your beliefs don’t say is that abusers don’t live by those rules. The general rule with abusers is they don’t admit what they did, and they don’t confess.
Now it is time to come to terms with what happened. A book which can help is Laura Davis’ ‘The Courage to Heal Workbook’. One of its central ideas is that confrontation is not required for healing. Another is, when you confront an abuser, expect to be called ‘crazy’ or ‘a liar’.
Your mother has called you both. It’s time to accept what happened and recognise your mother for who she is.