I'm going to call out my cousin for snapchatting with other guys when she already has a man.
Why do you think your cousin is doing this?
I'm not sure, but I know it's wrong for her to do this.
If you think your cousin is stepping out on her relationship by soliciting attention from other men, it's okay to talk to her about that behavior since it negatively affects her current relationship. However, it is important to do so privately and from a place of concern and not to publicly shame her since that will affect her reputation.
I am gonna call her out. I will post something to her friends telling them the truth.
While it's good to call her out, I would make this a private matter. Making it public will cause more issues than it will solve. Communicating this with her one-on-one will be the best thing to do.