I've tried digging around for a confirmation
From what I've gathered, Riot didn't actually plan for that to happen, it was just kind of coincidental - mind you, this was back in, like, 2013 (Grumpy Monkey is a Senior Character Artist at Riot)
Zerglinator:
I see the current arrangement of Shadow Isles champs as a correlation to the Seven Deadly Sins, where Evelynn represents Lust. I'd like her to work off of that theme if possible."
Grumpy Monkey:
Zerg, that's kinda cool. What is your list?"
Zerglinator:
Evelynn-Lust. Kind of obvious, being the Widowmaker. She's seductive, people believe she's a safe person, then she strikes when it's too late and leaves them no chance of escape. She always catches up to her victims.
Hecarim-Wrath. He's the Shadow of War, he charges in and starts destroying everything, and in a good number of cases, his furious charge leaves him with no option of running away. Once he begins his rampage, it's hard to stop.
Thresh-Gluttony. He devours souls all the time and keeps growing from it. He even has a hidden bonus buff when he reaches 255 souls which is called "Soul Glutton".
Yorick-Sloth. His lore seems to suggest that he rose from the dead with his job unfinished for a long time. Yorick doesn't do all the work himself, he mostly just summons Ghouls to do his work for him.
Mordekaiser-Envy. He drains life from people with every spell and turns it into a shield. His ultimate brings his envy of another's soul forth, stealing their health, their stats, and the entire person.
Elise-Greed. For the sake of her own power and vanity, she sacrifices tons of people to the Spider. Spiders also are commonly (to my knowledge) used to symbolize greed with their lifestyle.
Karthus-Pride. A man who thinks he's so important death should bow to him. Upon dying, his desire to keep himself alive allows him to defy death. His ultimate manifests his pride in his song, forcing all to hear his Requiem-it could even be the Requiem he made for his OWN death.
Tigernmas:
This isn't as relevant but I saw something earlier in this thread about the shadow isles champions representing the deadly sins, perhaps that idea is more useful if you apply it to the Darkin, with only one confirmed you could easily portray the rest in a way that they all line up with one sin or another.
Grumpy Monkey:
"Yeah I like the Seven Deadly Sin Theme.
I agree that making her a full fledged Darkin would be a lot of work, and could potentially close the door on making a brand new Darkin. Sooo, what if she was only Half-Darkin? A Hybrid with no place in the world, so she has to carve out her own destiny with all the fun character development that comes with it? I feel like I need to draw now."
This is pretty much all I could find on the subject.