、。
To tell the truth, I'm not your father.
Literal
True [possessive-の] thing [object-を] say [conditional-と], I [topic-は] you [possessive-の] father not.
A dramatic revelation using the 本当のことを言うと frame. お前 ('you,' rough/familiar) is typically used by fathers to sons or between close male peers. The sentence echoes famous fiction tropes about hidden parentage.