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Is Thorfinn really Thors' biological son? The soldier's remark "doesn't look a bit like him"(which seems obviously true enough from the colour of their hair) and Thors' instruction in the first flashback when he left the village to tell Thorfinn something important, makes me suspect that the answer is no, unless I'm missing something obvious.
Ylfa seems tough enough to be a good romantic match for Thorfinn if they don't turn out to be real siblings, although I suspect my speculative mind is probably wandering a little too much.