Heyyy it's been a while LMAOO sorry I don't really go on here anymore BUT if you want to read more, I'd recommend looking the web novel up on ncode.syosetu.com (you can just google 聖女の加護を双子の妹に奪われたので旅に出ます) and just using google translate.
As for spoilers, I can't recall a lot bc it's been a while BUT I'll do my best (I'll focus more on Lily's side of things)
But first: the ML is Heart, which I don't like bc wtf he was a grown ass man (or at least like a teenager) when he first met Marie
Anyway, here we gooo:
Lily is GENUINELY an airhead, but she's not actually malicious at all. She's incredibly spoiled, selfish, even stupid, and you sort of get the sense that she's never TRULY grown up past childhood but by the end I just felt really sorry for her. Like yeah she's done bad things, REALLY bad things, but man,,, I can't help but wonder what it would've been like if things were different.
Excerpt from Lily: "When I admire someone, I want to become that person. So I want everything about them. I hate that about myself."
The father ends up having a familial bond with Lily and Marie's childhood friend, and sees him as a son. He becomes a pretty good father figure!! Just not that good of a father figure to his actual children, which sucks ofc.
The mother has the tendency to,,, avoid things when they become a hassle. The grandfather sent letter(s) to Lily but the mother threw them away + I think she stopped sending letters about Lily as well when Marie was sent over. She has like zero growth.

Honestlyyy, the characterization is a LOT more nuanced than you'd think. I ended up really liking Lily and their dad (the mom not as much).
And yes, you read that right. LILY, the one who stole Marie's blessing, and THEIR DAD, the one who got pissed off that he wasn't going to be the father of a saintess. I ended up liking them MORE than the mom.
I LOVE me some complex character arcs.