How my Cousin See my Facebook Post

I added my cousin to the custom privacy setting so she can't see my posts on facebook because she is annoying like crazy and I don't even use facebook that much! so I updated my status and she comments on it something stupid and I just don't understand how she still saw my post, i tripled checked she was on custom ... it doesn't make sense how she saw my post?

Not sure about this but isn't custom privacy messages private just between you and her. If you posted on someone else facebook that is open to all

She could have seen it because you and your cousin have mutual friends. You may have customized her so she couldn't see your post, but your friends may not have.
A good rule of thumb of posting is: If you don't want EVERYONE to see something, don't post it. Things have a way of getting around.