Yes if we take away the fact that in FE4 there is no official pairing and that nintendo stated one time that Nanna is Beo's daughter in FE5. As far as we know Finn can (and is apparently intended to) be only Nanna's foster father.
Kotor3fan said: Nanna is Fin and Lachesis child. Delmund is Lachesis and Beowulf's Kid.
sartek said: If it's based on FE4 it could perfectly be Delmud.
True. The look Fin face reads 'This baby does not even look like me.'
Ony reason I tagged it as Nanna and "Fahter/Mother and Daughter" is I assumed it was based off of the fact that Finn could had fathered Nanna and i thought the baby looks like a girl.