@lenagirl11,
This is a tough one: you say he was there, he now denies it.
Some people black out in traumatic situations. Maybe he was there physically but the scene like that is so overwhelming that perhaps he numbed out., Maybe not even sure what he saw. Maybe he is trying to placate you and agree with you about what happened. Maybe he is truthful when he says he didn’t see it. Or maybe he was protecting you when you two talked about it days before.
You say he is now protecting your mother. Did you talk to him about that? If so he may say it’s because he doesn’t want to get involved between the two of you.
For your validation I suggest that you go to professional counseling and then bring your mother into it later. Is this one of many such acts that she did to you?
People rationalize their awful behavior. My mother was an alcoholic. Many times I had to get her off the floor after passing out and rescue her financially when she was in her 50s 60s and 70s. By the 80s she denied that she ever even drank! There was no use arguing with her at that point. And what good would it have done?
If nothing else your feelings need to be validated. Please sit down and talk with someone about this.