Well for the US, Maggie doesn't appear to have any documentation. Like a Social Security Number or a Birth Certificate. DNA testing could confirm paternity. And all of that presumes court proceedings to establish the facts.
All it takes is a call to sic the Family Court on you, but we don't know why anybody would do that. Do we? /s Harry wouldn't do well in family court. Now I don't think that the that members of the Mafia ever had their children taken away from them, but they have ended up in prison.
@Mira
In Blood Rites Eb pretty much puts the blame on himself for the way Maggie turned out. But since Jim has baked in Choice as a primary plot device in the DF, Maggie always had one. He also changed it up enough so that Harry turned out differently.
Also as an aside, if you were raised in a dysfunctional family like me, this dialog rings true. You know everything that can make the other person bleed and when things get hot you start launching missiles at weak spots.