Is Erin really Frank's child?

    
Blue Bloods (2010–…): Season 1, Episode 20 - All That Glitters -

Frank talking about Erin

Frank: She's successful because she's so hard on herself, but she pays dearly for it.

Frank: And I still worry about her like she's...

Henry: Like she's your own kid.

Frank: Does that ever go away?

Henry: Not really.

Is Erin not his child?

Never saw anything else about this or am I missing something?