Been pondering the fact of both the themes of S3 and S4 of Hazbin Hotel being described as family and just thinking. If Eve/Roo really is Charlie’s biological mother, that would make her Cain and Abel’s little sister. And I just. I think Cain/Abel/Charlie as a sibling trio has the potential to foil Arackniss/Angel Dust/Molly fantastically. The early concept art implies Arackniss killed Angel, like how Cain killed Abel:
(though tbf because this is early concept stuff Arackniss killing Angel Dust might have changed? If I recall this is from the same era as when Vivzie was planning for Angel Dust and Vaggie to be romantically involved? So a lot of stuff has clearly changed/evolved since then, so take this with a grain of salt)
Other early concept art and animation like “Daddy Issues” shows Arackniss being more like his father and favored as such, while Angel was seen as a disappointment and didn’t measure up to his father’s standards of masculinity (which matches Abel’s own implied relationship with Cain and Adam, with Adam not liking Abel, Cain being the “braver son”, and Adam apparently playing favorites with his sons, with Abel probably not being the favorite).
Angel and Molly also being twins, while Abel and Charlie are not literal twins, are both blonde and very similar in terms of personality of being conflict avoidant/people pleasing/starved for approval Miss Rainbows and Mr Sunshine. We see sunburst behind Charlie and Abel here:
And rainbows are associated quite often for Charlie, like in her songs and drawings, while rainbows also appear in Abel’s trading card:
We don’t know much about what Molly is like, but based on her being in heaven, her cheerful appearance and her name likely being a reference to the drug Molly which is ecstasy, (which makes the user experience intense positive emotions of pleasure and empathy) it’s easy to draw foiling between Molly/Charlie being the most positive and optimistic of the siblings, who just want to make everyone happy. I also think it would be sweet if Charlie reminds Angel Dust of his sister he misses, and is part of the reason he has a soft spot for her, because they both have love and have unwavering faith in him.
Both also have a little marching band motif going on, with Abel wearing a marching band uniform, and Charlie describing herself as “I can be the marshal leading the parade” during Ready for This, with Rosie (who might also be Eve/Roo) handing her parade stick to her.
Arackniss and Cain likely are the “darkest” and more corrupt eldest sibling, with Charlie and Molly being the “brighter” and more good sibling as the youngest. And then Angel Dust and Abel caught somewhere between the light and dark as the middle sibling, being more morally grey.