Somehow, I think that, in context, this would make even less sense.

C'mon, Grandpa, go with it! If he's mentally challenged, you'll just frustrate him trying to bend him to the truth. Use your superior intellect to improvise his vision of his world. It'll be fun for you and make him happy. Give him Life Savers and take him to football games. That's what my grandpas did for me.

I think we need some context

Sounds like a challenging conversation. - No, uncle, come here, sit on my lap and I'll tell you that you're not african-american, I'm not your grandfather, and you're a bit of a challenge for the rest of us... I wish you the best of luck!🤗

Sounds like the opposite version of Chappelle’s Show skit about Clayton Bigsby, a blind black man who also happened to be a leader in the white-supremacy movement. Only in this version your white uncle aspires to be a black panther… OK, the real situation isn’t really funny. I assume Uncle either has dementia or is seriously mentally challenged. Frankly I’m not so sure it’s worth the effort trying to convince someone of the truth if they are incapable of learning, understanding, and retaining that information. Sometimes it’s probably just easier to humor the delusions if they are incapable of understanding and retaining the information. I assume you know if Uncle has always been like this or if it’s dementia. I am very sorry, I cannot imagine how awful this is to deal with. My step mother had Alzheimer's. It came on slowly but inevitably. Fortunately no one in my genetic heritage had that - It sometimes runs in families.