Sorry, I'm confused, I don't get why Lalli's mum would have a swan luonto? Oh, duh, you meant Tuuli, Eino's wife, sorry.
But you bring up an interesting point about the love-heart swan motif. Do present-day ordinary Finns have a sense of what their own luonto is? That is, Eino having that knowledge prior to the development of the Rash and rise of magic? It's presented as worldbuilding stemmed in and extrapolated from actual Finnish culture. Is it a sort of thing similar to many saying, 'oh that's my Cancer coming out, always so emo haha' or 'gawd, that's an evil-twin Gemini for ya' even if they don't believe in astrology?