I know you're mostly into the Black family but i have an interesting take on Petunia and Lily you might like!
In HP, Lily's red hair and green eyes are described as positives by everyone. However, red hair was not seen as good or pretty in the UK during that time. It was seen as a sign of celtic blood and there were many people who were (sometimes viciously) bullied for it.
Lily, who grows up in a working class, who has bright red hair and can do all sorts of strange things, is 10000% not popular before Hogwarts.
Petunia on the other hand, with her blonde hair and blue eyes, would definitely have been considered the prettier sister and the normal one.
I am convinced that Lily was an outcast before Hogwarts. I think this is part of the reason Petunia and Lily are so close. Petunia is the only one who hangs out with Lily. (I also think Lily being an outcast is what causes her to befriend Snape and keep being friends with him after all the things he supposedly does)
This would also help further understand Petunia's switch up when Lily goes to Hogwarts. She specifically mentions their parents fawning over Lily's magic. I think Petunia was the "golden child" or the "normal" one until Lily gets her letter.
When the letter arrives, Mr and Mrs Evans finally get an answer for Lily and her strange red hair and her too bright green eyes. An answer which doesn't make them the bad guys (aka the people down the road who have celtic blood). Petunia loses a bit of her status once Lily actually goes to Hogwarts.
I'm certain I can write this in a nicer way but I was a bit too excited sharing this with the one mutual I have who appreciates canon!!! I hopr you enjoy it :))
I imagine Lily being a bit of an outcast at Hogwarts, too (while paradoxically appearing popular). Because of her relationship with Snape (a known blood supremist who calls people mudblood), the other muggleborns were probably wary of her, and she couldn't quite fit in with the purebloods.
I adore analysis of Lily :D