[@BlackPanther] it's rare, it's risky, and it results in greater power, which is generally undesirable for the purpose seasonborn were created to fill.

[@Ojo chan 42] The sister lived there, and even with Clyde just visiting there's nothing stopping Eleanor from saying something about it.  Or the sister from saying something to Clyde before her death -- I'd think she'd figure it out significantly before the date of her death, and Clyde was visiting frequently.  Unless there was considerable indiscretion (which is a possibility, I suppose), a relationship that has a child conceived has been around for some time.  I can't speak for how you wish to play her, but it seems likely a girl would speak to her mother-figure about the man she likes, particularly if he's not human and the mother is likewise the same sort of creature.  I also can't speak for how many years Clyde was gone looking for a way to turn Eleanor seasonborn.

My honest opinion is that the simplest way to handle everything would be to have Clyde find and grow interested in a certain not-really-human after Laira's father has abandoned her/her mother because of losing interest, but that would require her being seasonborn, and not a kelpie.  Or perhaps the mother deliberately stopped contraceptive measures in order to get the father to stay, and he resented that.  He liked her, he wanted her to live a long time, but he wasn't willing to stay with her all the time, nor to take her back to the fey realm permanently.  *shrug*  That's just how I'd write it though.  Your way is definitely a possibility, though a tricky one.  But that's the fun of writing, isn't it?

I think it is some sort of complicated ritual, but it risks harming the humans -- particularly with fewer of them being changed -- and it requires a good deal of power from the caster.  I think it's done over a period of time, to reduce risk of insanity, from what I remember of the discussion on the topic before.