Welcome to the PC forums! You will find a lot of support here.

I'm not familiar with FB, but I believe there is a blocking feature. Perhaps that is in order. It will remove a platform for your uncle's concerning comments. He may be grandstanding or he may be serious, I can't speculate on that.

You said he has mental problems and he certainly is upset. Therapy may help
him as well, as a visit to his doctor for an examination, if he is not already in treatment.

Wishing the best for you and your family.

If you have any questions or concerns, please contact a Community Liasion.