The plot is becoming more implausible as it progresses and the writing for the dialogue needs to improve, especially…
I would like to start off by saying that this is just my take. I would like to add to that by saying that we need to keep in mind that this is just the 5 ep and that more may come to light as it goes on. So I still have high hopes for this series. Now with that being said, I don't think that the plot is implausible yet, I think that the twins have more than one reason for them to switch "permanently" I would like to point out that we never got to see the full conversion between the twins. Just what Sprite told Thun. I agree with you, I am not too fond of the twins mother either. My opinion on the father is still out. Sam likes Zee but never got the chance to tell him, seeing as Sprite is now Zee there might come a confession later from Sam. But even so I think Zee likes Salmon, and Sprite as Zee is starting to or has from the beginning liked First. Going off ep 5 I would say that Sam and Thun have only really interacted 3 times. So logically Sam is going to be a bit reluctant in the beginning.