Not a lot of information but this might give you an idea.
From Peanuts to Powder Puffs
Jeannie Schulz
by Margaret O'Connell
ST: Why do you think Lucy grew up to be both the meanest and arguably the most important female character in the strip, when Violet and Patty were already there doing things like snubbing Charlie Brown and making a point of not giving him valentines when Lucy was still being drawn as if she were in nursery school?
JS: I thought your [advance] question "Does the fact that she was the first one in the group to get a younger sibling to deal with have anything to do with it?" I have a note "Sparky would like that question." For some reason it's hard to know [why] when you go back over the early '50's, Violet almost morphed into Lucy.
ST: They even look alike.
JS: Yes, exactly. So I don't really know why it didn't stay Violet. But I think you're right, or you're going in the right direction, when you suggest that because she was the one who had the brother, then you have a combination where she can not only be crabby and sarcastic to Charlie Brown, but she can also then lord it over her little brother. So it then becomes a little more complex a situation, whereas Violet was just Violet. So I think you're onto something there, and that once a character had a little brother, that character evoked more possibilities for funny things. So I think that's a good observation.
This is taken from another site:
Patty and Violet, really never known for more than picking
on poor Charlie Brown, weren't doing anything that Lucy
couldn't take care of by herself...and so they gradually faded
into the background.
