I mean her methods are not the cleanest, but if you play through her flashback and confession again, when Helga goes away for a morning after the massage (2.6 I think?), you can see Lyna simply has a trauma and that's the root of everything.
The bratty girl just wants a full "wholesome" family she never had, and incest is the most extreme love. She was taught to be very stubborn after all...
I forget where in the story but it is indeed clear that she's got traumas.
The whole family is fucked up since square one. Lyna went throu life getting attention mostly by acting up, leaving her starved for unconditional affection. Eric learned to show affection by pleasing others, Meating the conditions mostly at his own detriment. Helga learned from her past to always hold control over her situation lest it cost her another family connection. Thus making her affection conditional so it doesn't cross that boundry again. Falling into incest is max affection with the easiest condition for them all, simply being related.
This is all clear proof you got these characters properly realized and clearly presented.
Doesn't change that the VN carries Lyna's icky bias. But I do prefer my incest stories to not pretend that it is mentally healthy even when it's better then the hell they characters already got. It's the difference between explaining it, justifying it and glorifying it. here it feels like it hovers between the later two. This was the reason I had to pick up this game again after the initial squeek made me delete it round chapter 1 ~ 1.5. Why Helga's my fav (aside from being a dommy mommy) is that she's firmly aware of the consequences and refuses to budge on it being wrong first, hot second. May she win the battle of wills against Lyna.
DomMom Supremacy
(low key unrelated: why do the songs emphasize how nordic Lyna is? What's it ment to add to the storytelling?)