I just found out my cousin is adopted and I want to tell her.
Why? What's going on with your cousin?
I heard that her parents are not her real biological parents.
Telling a person that they are adopted is very upsetting and emotionally traumatic to that person if he or she does not already know. It's very wrong to try to purposely hurt someone else's feelings. You should avoid talking to your cousin about her adoption status as that is very sensitive information and is something that should be disclosed to her only by her adoptive parents.
It's still true that I want to tell her.
Sometimes you just need to keep a nonharmful family secret. If you say something it might cause doubt and shame and she could possibly hate you for a while.