My grandma thinks grandpa is cheating, what can I do to make her realize it's false?

She isn't thinking clearly and isn't recognizing me anymore. She really thinks (even believes) grandpa is cheating on her with me. I stopped by for a visit to help out and she thinks I'm his mistress. She has called him a dirty, bastard and is acting devastated that he can throw it away for a young woman.

She's deteriorating and this is hurting the rest of the family members.

My grandma thinks grandpa is cheating, what can I do to make her realize it's false?
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