Mom is blaming me for cheating allegations?

She has been accusing my dad of cheating at his job 2 years ago. From then she’s been mad at me and separating me and my sister from my dad because we won’t accuse him too? It makes me sad and I don’t understand where she’s coming from. Me and my sister live our own lives. Why are we treated like we did something wrong because we won’t fight our dad for her? Has this ever happened to someone? I’m so confused about this? I don’t believe my dad cheated either but she insists over and over. She claims we take our dads side and won’t believe her. I’m not sure how my dad could cheat if they are always together. The been married 40 years. Is there mental illness going on with my mom?
Mom is blaming me for cheating allegations?
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