I am not planning on telling my cousin about some family members being slave owners who owned other family members.
That seems complicated. Why do you want to hold this fact from your cousin?
I did not want to tell anyone about this secret. It's a pain in ass that people know at school.
I'm sorry you had to find out about such a terrible secret, as slavery is one of the worst parts of US history. Sometimes not knowing the truth is best, especially since this probably happened way too long ago. You don't want to mess up your family relationships.
It's not like anyone forced us to commit a crime. And I do want my cousin to know.
Then if you feel that he will be okay with it then tell him but it is okay if you think it will hurt him to keep it from him. You don't want to hurt him just to get if off your chest.