If it’s an extended family member, it could be anyone, but I’m guessing that Tom’s young daughter and his dumbass drug-dealing son aren’t harboring some secret identities.
Obviously the outlet is being teasy about the big twist, and is only saying that one family member isn’t exactly the person they are saying they are.
It only took Kirkman’s bodyguard a few minutes to find the kid on the night of the terrorist attack, after all.
A drug-dealing side gig is hardly a major identity secret.