Something similar happened to me when I was a kid but I want to explain how easily you can be manipulated as a child. Growing up my dad used to constantly negatively put down my grandmother in front of me (on my mums side) even though she never did anything objectively bad to me. It got to the point where I believed him and didn't like her at all and found her a total nuisance. Years later as an adult, I see how abusive and violent my dad is and how much sincere love my grandmother had for me. She never once said a bad word about my dad even though she knew he was talking so much shit behind her back.

All of this to say, even someone as kind as my grandmother, children will think are evil if someone just puts it in their head that they are. But now you are an adult and know the whole truth, you have the option and power to choose what you want to do with the truth. It is less simple for you because it appears your mother might not be the most loving or stable, but nonetheless you now know the truth.

If I was your mother, it would feel good for my child to at least recognise what I said was true. However, what comes first is YOUR mental health and protection. Keep your boundaries up for your mum and your dad. First you need to come to terms with the truth THEN figure out what you want to do with the truth.