Because why would EJ acuse Ari? That gets him nowhere, but looking stupid. I am sorry, if it is EJ, then he would be acting differently. He would "act" upset and go after leads with others, but he would not accuse random people that no one suspects.
It could be Kate working with Anna, Anna working on her own, or like someone else said a long time ago, maybe Tony is alive? Maybe Anna doesn’t realize it is Sami’s child, because I think she is trying to hurt Stephano, not the Brady’s.