My theory at this point is it will be Ronnie who finds out that Jack is the father and she will then reveal it to Sean just to twist the knife in. Roxie's betrayal will be total to her and she'll want to get her back not only for shagging Jack but for leaving her.
My theory is that it is poor, stupid TANYA who finds out that Jack is Roxie's baby's daddy. Her only option is to kill the following, in this order:
Max (again),
Jack (for being a bastard), and
Sean, well just on principle.
