In the ep "Burked" (some scenes cut for relevent ones).
SAM BRAUN: I would've taken you home right then if I could have.
CATHERINE: I know, but you were married and I was a baby. It would've never worked.
At the end of the show
SAM BRAUN: You know, I should've married your mother.
CATHERINE: Well, considering I was six months old when you guys lit the flame ... a lot of time has passed. You had plenty of chances.
SAM BRAUN: Just wasn't in the cards, Catherine. It just wasn't in the cards.
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The Scene From ITB I cut off part of the scenes to get to the relevent parts.
CATHERINE: I guess I didn't have a good enough reason until now, but ... I tested my DNA against the unknown sample. (She sighs.) -- And wouldn't you know ... there were seven alleles in common.
(He leans forward and shakes his head.)
SAM BRAUN: That supposed to mean something?
CATHERINE: It means that you and I are biologically related.
SAM BRAUN: Do you really think I could murder someone?
CATHERINE: Science ... tells me that you did. So, yes, I do. (Her voice breaks.) It's just such a lousy way to find out that you're my father.