I'm going to teach sex education to my younger cousin
What do you plan on saying?
I'm going to tell her all the details about what's happening when boys and girls get to know each other.
Sex education is best between a parent and a child. It's inappropriate to take the place of a parent to discuss sexual matters with a child, especially without their permisssion.
Why?
You could be seen as grooming a child for sex. Let their parents handle this, it can go really wrong and you might be accused of something illegal.